Newsnight Live - 20 April

With the launch of the SNP manifesto today, I thought it was time to do a quick audit of the all-important music used by each of the parties at these events.

First up were the Tories, who went classic with Fleetwood Mac's Don't Stop Thinking About Tomorrow which was famously used by Bill Clinton in 1992 and prompted the band to reunite briefly.

It's a safer choice than at the party's 2010 manifesto launch, when their choice of Keane's Everybody's Changing prompted the band's drummer to tweet that they were "horrified".

My job got a little harder when it came to Labour's launch. I listened, I Shazamed, I googled lyrics, I made increasingly plaintive calls to the party press office, but I still wasn't able to identify the track that the Shadow Cabinet walked on stage to . If you recognise it - do let me know.

An easier find was the tune that Ed Miliband left to - Damon Albarn's Mr Tembo .

One track that wasn't an option for Labour was D:ream's Things Can Only Get Better - famously used by Tony Blair in 1997. Band member Brian Cox recently said he wouldn't let Ed Miliband use the track.

The Lib Dem manifesto launch prompted another wave of calls to the press office, who could only tell me that Nick Clegg left the stage to Harder Than You Think by Public Enemy - previously used by Channel 4 for its Paralympics coverage.

I had to play detective again after watching the SNP's launch. A helpful press officer got back to me quickly to explain that the track was called Metropolis by an outfit called Ded Good. I had absolutely no success in tracking it down online but you can have a little listen here .

And as for UKIP? They didn't use any music for their manifesto launch. Not even Purple Reign (sic).

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In tonight’s interview with Evan Davis, Ed Miliband was at pains to stress that he wasn’t going to get into answering hypothetical questions.

Except Ed Miliband has already said he won’t go into a coalition with the SNP. Hypothetically speaking, that is. So what’s the difference between telling us that and not telling us what life would be like if he ran a minority Government but with SNP support ?

The answer, of course, is that one hypothetical is a message he wants the electorate to hear. The other is one he’d rather people kindly ignore.

Ed Miliband is not alone in avoiding the hypothetical question. Politicians from all parties do it. David Cameron has refused to discuss the possibility of any post-election deal with Ukip, for example.

The problem though for Messrs Miliband and Cameron is that according to Chris Hanretty, who produces the Newsnight Index, the probability of no majority for either party is currently sitting at 92%.

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So "no overall majority" remains a hypothetical outcome but a pretty darn likely one. So we ask these hypothetical questions. They are attempts to get at the biggest hypothetical of all: will politicians deliver in Government what they promise in the campaign ?

Amid the spreadsheet-fiddling, I discovered one big curiosity today. The SNP has sought to invent a new way of adding up. In this document, a formal Scottish Government reply to a formal Treasury costing, the party has set out a clever new way to account for the national debt.

It notes the Treasury method:

• "The Treasury analysis adds the debt, arising from the proposed Scottish Government approach, in the same year as the borrowing occurs."

And the Scottish Government method:

• "These projections use a transparent and straightforward approach to allocate debt, as a result of the extra spending proposed by the Scottish Government, in the year following the borrowing."

The SNP is using this accounting approach. It means that it has started delaying the moment at which it scores debt for a year. But you owe debt when you owe debt. There's really no good economic rationale for claiming otherwise. This is simply a bit of a dodge.

Today, the SNP promised "a real alternative to the pain of austerity, an end to unfair policies like the Bedroom Tax, a higher minimum wage and protection for our NHS and vital public services".

The only firm budget commitment in the document is to increase spending by the equivalent of increasing the size of one major budget line by 0.5% each year in real terms. In the lingo, that budget line is "TDEL" - total departmental expenditure.

If we assume that this is the size of the SNP's proposed extra spending (and their manifesto is vague on this) what does it tell us? Well, it's quite a bit more than the Tories would intend to spend, but maybe doesn't quite live up to the SNP's rhetoric about holding Labour hard to the left.

Could the SNP and Labour agree to a programme? Yes. The SNP 0.5% plan could actually lead to spending less than Labour would spend if it were to hold off on balancing the day-to-day budget until 2019-20.

They do, however, imply the SNP might spend a bit more than Labour if it were to try to balance the current budget in 2018-19, and then freeze the budget in 2019-20 - although that is quite a conservative set of assumptions.

The tables for total nominal public spending by the parties, on those assumptions, looks like this:

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Tonight's Newsnight index shows Labour losing ground as the Conservatives slowly build up their lead - though both parties are still well clear of a majority. However it is the SNP that have gained the most. They are now forecast to take all but 12 seats in Scotland.

For the course of the general election campaign, Newsnight each evening will be publishing an exclusive Newsnight Index on the likely outcome, based on a sophisticated forecast model. It is produced by Professor Chris Hanretty from the University of East Anglia and his colleagues at electionforecast.co.uk.

For more information on how the Index is produced, see here

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Ed Miliband’s enthusiasm for all things American is well documented. Where most of the New Labour tribe underlined their demotic credentials by their mastery of football trivia, Ed preferred the nerdier milieu of the baseball fan, once reportedly bunking off during a climate summit to watch his beloved Red Sox play.

In a 2010 interview he said his favourite book was Michael Lewis’s The Blind Side, a study of American football coaching strategy. His political hero is Bobbie Kennedy.

Recently there have been more signs that his stint lecturing at Harvard in the early 90s – and an earlier spell living with his father while he taught at Boston University – left a lasting impression. In his Sky/Channel 4 election interview he deployed the distinctly un-British formulation “Hell yes!” to allay the suggestion that he might not be tough enough to be PM.

For his interview with Evan Davis today he drew on the Hollywood hard man’s phrasebook once again.

Would Nicola Sturgeon call the shots in a future Labour-led government?” Evan asked him. “That ain’t gonna happen,” Ed replied, adding, for the avoidance of doubt: “That ain’t gonna happen.” He wasn’t actually chewing tobacco at the time but he may well have been.

Another of Ed’s answers perhaps betrayed the influence of the West Wing on a generation of Labour politicians.

Pressed by Evan on the constitutional implications of a hung parliament he declined to have “an inside the Beltway discussion about permutations and hypotheticals ”. The phrase refers to the highway that circles Washington DC – and roughly translates as “inside the political village”. The irony, of course, is that only Beltway types - or avid West Wing fans - are likely to know that.

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Normally, we expect the most interesting parts of an interview to be where an interviewee says something interesting.

Well, Ed Miliband turned that rule upside down today in my half hour encounter with him this morning. (This was the third of our Leader Interviews, and you can see it on BBC One at 19:30 this evening and a 10 minute version on Newsnight). The most revealing parts of the interview were where he said nothing.

For example, I had hoped to spend about eight minutes on the constitutional issues arising out of a hung parliament. In the event, we didn’t need that long. I asked him whether the biggest party in the Commons has a moral right to govern. Mr Miliband batted it away with the words “I’m really sorry to be disappointing about this but I’m afraid I’m not going to get into post election speculation”.

I had another go, but didn’t really progress. I asked whether, if his party ends up ruling England only with the help of the SNP, will there be all hell to pay? Again Mr Miliband batted it away with those famous words, “I don’t do hypotheticals.”

He was similarly unforthcoming on his plan for the deficit – or the lack of a specific plan for where he expects it to be at the end of the parliament. I tried to break new ground, but the ground remained unbroken.

I don’t blame the Labour leader for obfuscating on these issues. He doesn’t want to risk igniting an almighty row at this point in his campaign, particularly as it seems to be going well for him. But my rule of thumb is, if you want to know where they feel exposed, try looking for the issues they don’t want to talk about.

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Today's SNP manifesto launch was a seriously good piece of political theatre. In a rocky auditorium, Europe's largest climbing wall, filled with 400 or 500 assembled supporters, signs declared "embrace the fear".

We all wondered how she would make her entrance - so many options with harnesses hanging above the stage.

In the end she came from the back of the cavernous arena. During the warm up act with Stewart Hosie, a spontaneous roar of applause broke out as people realised Sturgeon was there, unannounced and just a pin prick. Rarely in politics do you see such (minimally stage managed) excitement.

Her husband, the SNP's chief executive Peter Murrell, chose the venue. "It was just a no brainer", a source said. "Much better than a dark nightclub in Battersea" (a reference to the Lib Dem manifesto launch).

In the event itself Sturgeon was shrewdly amusing when asked curve balls like "why do the English hate you?". But she was less expansive when asked the tricky question of the day: Just what negotiating power does she have?

Towards the end of last week senior Labour sources were briefing that they would not have to yield to the SNP at all in any Budget negotiations. Because the SNP had made clear they wouldn't go in with the Tories, they have nowhere else to go.

The SNP's view this weekend is that under the Fixed Term Parliament Act, voting down a Budget is no longer a confidence issue, so it doesn't bring in a Tory government. We broke this line on Newsnight's Friday programme.

The question then becomes: Could Ed Miliband decide to make this issue one of confidence? Today I asked Nicola Sturgeon this, along with another similar question. She refused to go there.

It had seemed that today the party was dampening down the idea they could kill Labour budgets... David Cameron used evocative and probably quite effective language of the SNP holding Labour to ransom.

But I understand that a national newspaper will this evening carry an interview with Nicola Sturgeon where she chooses to explain how the SNP could block Labour budgets in a future Parliament.

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In Evan's interview with Ed Miliband (full version on BBC1 at 7.30pm, shorter version on Newsnight, usual time and place), he asked the Labour leader about whether he could assure voters that Labour were not planning a top-down reorganisation of the NHS. He said one word: yes.

While it's always refreshing to get a clear and straightforward answer to a question, if you look at what Labour are promising, they are pledging to scrap the Health and Social Care Act, which the Coalition sweated blood to get into law, not without considerable controversy and collateral damage to their reputation on the NHS.

The Labour manifesto says "we will repeal the Health and Social Care Act, scrap the competition regime. Restore proper democratic accountability and establish a sensible commissioning framework".

Sounds to the layperson like quite a bit of upheaval, and it's not just Tories and Lib Dems saying that. The Health Foundation, an independent charity which looks at health policy, says in its analysis that "There needs to be more detail as to how Labour will manage to achieve commissioning for a single health and care service without further top-down reorganisation."

In the interview, Mr Miliband goes on to say that it can be achieved by integrating services properly. The details, he said, can be found in a report by Sir John Oldham. You can read that report here.

However, that report too has been subjected to scrutiny by experts. Richard Humphries, Assistant Director at the respected health think tank the King's Fund, said this about the reorganisation point:

"But there are vital elements of Labour’s vision that still remain cloudy. If the intention is to de-clutter the organisational landscape by having fewer (but more integrated) organisations, how will be that achieved without further reorganisation? If health and wellbeing boards evolve into the single commissioner, what role does that leave for CCGs? It defies credulity that change on the scale implied by the plan can be achieved with no organisational change at all. The definition of ‘reorganisation’ looks set to acquire unprecedented elasticity."

Mr Miliband may well be correct that he can achieve his goals without the huge upheaval that caused so many problems for David Cameron in 2011. If not, then his very straightforward response today may end up causing him a lot of grief if he ends up as prime minister.

We played Ed Miliband a clip today, when Evan interviewed him for the latest in our leader interviews, of Margaret Thatcher claiming that the Labour party were willing to sacrifice growth for everyone on the altar of equality - in other words, that they wanted everyone to be worse off so long as they were more equal. This was his response:

“The old view is that inequality could be better for everyone – better for the richest because they were richer, and better for the poorest because they were richer too. It doesn’t work like that.”

Ed Miliband, famously a bit of a wonk, seemed to rather enjoy talking about this – I suspect this, from the well respected OECD, is the evidence he was referring to:

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I appreciate the OECD’s graph here isn’t entirely clear – but basically, they believe that there is evidence that inequality itself does actually hurt growth. The orange bars below the x axis show you by how much they reckon it hurt growth in each country in the period they’re looking at. The OECD also suggest that the effect of redistributive policies per se is actually roughly neutral on growth.

So, game set and match to Miliband then? Not quite.

This is a hotly contested area – and to illustrate the other side of it, a lot of economists (perhaps even Thatcher) might cite Okun’s leaky bucket. In Okun’s own words, the problem with government redistributing is that:

“The money must be carried from the rich to the poor in a leaky bucket. Some of it will simply disappear in transit, so the poor will not receive all the money that is taken from the rich.”

As well as the sheer administrative inefficiency of government, Okun adds that people on low incomes lose incentives to work if the government gives them money, and the rich have an incentive to dodge tax:

“High tax rates,” wrote Okun, “are followed by attempts of ingenious men to beat them as surely as snow is followed by little boys on sleds.”

Something that all parties, with the billions they are hoping to recover from crackdowns on tax avoidance, seem to be acutely aware of at the moment.

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Slowly but surely the notes I receive everyday from asset managers, economic consultancies, research firms and investment banks have turned their attention to the UK election. The latest is from Morgan Stanley and is broadly representative of the apparent emerging consensus.

They see three possible post-election governments (as illustrated above):

1. A fiscally responsible, market friendly, eurosceptic government - led by the Tories.

2. A fiscally responsible, euro-friendly, intervener - led by Labour.

3. An anti-austerity, euro-friendly, intervener - Labour plus the SNP.

They aren't especially keen on any of them: "We see the impact on the economy as negative on all scenarios", Morgan Stanley say.

But before we get carried with gloom, it's worth noting that they add: "Nonetheless, across all scenarios, we see growth continuing at around 2% in 2015-16, and the deficit more than halving over the next Parliament."

In other words, whoever forms a government, Morgan Stanley don't see the UK recovery being derailed.

Postal votes are going out this week (you must submit any postal application forms to your local electoral register by Tuesday 5pm!), so some of us could be voting as early as today. As the chart below makes clear, this voting method is becoming increasingly popular, so time really is running out for the parties to make their pitch.

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Does the increase in postal voting deflate the importance of "polling day"? There’s no record that we know of that tells us the number of people who voted in 2010 before Election Day. But that hasn’t stopped David Cowling, the BBC’s Policy Research Unit editor. He’s number crunched some recent studies into this and found that:

• In the 2003 local elections, 64% of postal votes had been returned a full week before polling day.

• The 2004 local and European elections saw the largest study into postal voting. On the Thursday before polling day, some 34.7% of postal ballots had been returned.

• In the 2013 Eastleigh by-election, 60.7% of postal ballots were returned a week "early", a figure comparable to the 2003 study.

If we take the more conservative 2004 figures and crudely apply it to the 2010 General Election, then we see that by the Monday before election day, almost 4.4 million had been cast. That’s by no means an exact science of course.

What impact could this have on the elections? Well, as my colleague James Clayton argues postal voting could be a key factor in the SNP’s rather late manifesto launch date. More generally, the more people vote this way, the shorter the campaign period becomes and, in this case, the less time Labour and the Conservatives have to break open the polling deadlock.

A final point. It might lack the romanticism of walking into a booth with your polling card, but postal voters appear to be far more likely to vote than the rest of us. In 2010, 83.2% of those with a postal ballot returned it, while the turnout for those voting in-person was just over six in 10.

I was just glancing at the forecasts over at the website of Election Forecast, the team who compile the Newsnight Index.

Their broad forecast is outlined below.

What's really striking at the moment is that two words - "holding steady" - now define the performance of all of the major parties. We're three weeks into the campaign and, so far at least, not much is moving.

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Nobody thought that defence would be an election issue, but it is – and the SNP manifesto restates that the nationalists will have no truck with Trident renewal.

But there’s another little snippet in the manifesto that will have wry smiles breaking out among officials in Downing Street and the Ministry of Defence.

It’s this…

"There should be no further erosion of the Scottish regiments and it remains our ambition to see the traditional regiments restored."

That might prove difficult. The Scottish Regiment took a hit when the Army made cuts in 2012. But that blow was softened by...David Cameron.

Some of those who were involved in the discussions at the time say the Army wanted to go much further north of the border, pointing to the problems of recruitment and retention.

But Downing Street didn’t want to give the nationalists a free hit ahead of the referendum, so the top brass backed off.

Cue lots of grumbling inside the MoD about the cuts being used as a political football. Grumbling that continues to this day.

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Viewers who watched our Election Late Show on Friday night may have noticed James O'Brien's cursory reference to “the people politicians insist on calling ordinary”.

It was of course intended as a comment on the way our political classes rather presumptuously categorise their voters (ordinary is defined as with no special or distinctive features...)

As you can see below, this apparently wasn’t enough to escape at least one viewer’s ire.

Journalists covering the election campaign: consider yourselves warned.

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The SNP manifesto advocates cancelling Trident and a "defence that works for Scotland". Taken individually many of its suggestions, from giving Parliament a veto over any future UK military action to introducing new maritime patrol aircraft, might find favour with military or foreign policy experts.

Overall though,like many left or progressive parties,the SNP's struggle to make an active case for defence - for example in terms of possible threats to the British Isles - since that might be politically distasteful to its activists. Instead the defence budget is to be used as a cash cow: the money from a cancelled Trident to be "invested instead in better childcare, education and the NHS".

SNP manifesto writers bemoan military cuts because they have resulted in "fewer jobs and less defence related spending" in Scotland; and they urge for the breakdown of the national defence budget into national regions because of the party's belief that "a far larger proportion of the defence procurement budget should be spent in Scotland".

Of course even under a 'Devo Max' separation of powers, the UK government retains control over foreign affairs and defence. But that hasn't stopped the SNP putting forward its views on those subjects. It isn't made clear though why they think any money should be spent on the military (aside from benefiting Scotland's economy). Today's manifesto does hint, "our near neighbourhood including the High North and Arctic are a key priority for Scotland", and notes the importance of cyber warfare. But why, what's it for? There is no mention of Vladimir Putin, the Islamic State, turmoil in Libya or any number of other issues on Europe's periphery.

The idea of isolation in Europe resulting from Brexit clearly worries the SNP, as it does many south of the border. And the party alludes to Scotland's history of internationalism. But when it comes to the cash involved in defence,the party seems cleave closely to the 18th Century view; that its prime purpose is for patronage.

Even as EU leaders are meeting in Luxemburg, more news of migrant deaths at sea has come in. Our reporter Secunder Kermani is in Lampedusa, where he's been filming in a holding centre for refugees. We'll ask what possible solutions there are to this situation.

Evan Davis has the next in his series of leader's interviews: tonight you'll see his conversation with Ed Miliband. And Jonathan Freedland, Daniel Finkelstein and Sarah Sands are on hand to give you their thoughts on Labour's election.

Allegra has been at the SNP launch today - with no symbolism at all, it was held at an enormous climbing wall.

And David Grossman has taken to the twittersphere to analyse the election. The results, it turns out, look rather like Star Wars.

We are now three weeks into a (just less than) six week election campaign. And, despite the fire and fury from the parties, very little seems to have changed in the polls.

I wrote last week about why you should be careful of individual polls that show big changes in the vote because of sample error. In that post, I said that wiser heads would be looking at polling averages - polls of polls - to see what the real trend was.

But even these are fallible.

Take a look at this chart. The thin blue line shows the Conservative lead over Labour in a rolling one week average of polls - one of those 'polls of polls' I was talking about. If it goes below 0, Labour are ahead and vice versa. Each vertical line represents the start of a new week. Tell me what you think the story of the campaign is:

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Looks like a marginal, but robust Labour lead doesn't it? Now look at this chart:

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Looks like the Conservatives are, marginally, beginning to pull ahead doesn't it?

The punchline is, BOTH these charts are fake. And BOTH are built on the assumption that the Conservatives and Labour are IN A DEAD HEAT. As with my last post, I asked my computer to take the underlying assumption that the Tories and Labour enjoyed exactly the same level of support - then 'poll' my virtual population just as polling companies would. The simulation then calculates a poll of polls from these results, just as many psephologists do. Again, those trends you think you're seeing are literally pure statistical noise - there is no actual lead for either party because the whole experiment was based on precisely that assumption.

Why does this matter? Because, over the next couple of weeks, people will be watching for shifts in the polls of polls very, very carefully. When you see them moving in one direction for a while remember these charts. The parties are more or less in a dead heat in most of them at the moment. My advice would be not to take any notice of moves in an average until it is at LEAST 1 point or, to be comfortable, 2 points different from before. Even polls of polls can mislead.

So, if we can't trust polls and we can't trust small movements in polls of polls, how can we know who's going to win? The answer is that, with things as tight as they are, we simply cannot.

tweets:

Quote Message: Been watching Sturgeon's husband @PeterMurrell carting boxes of manifestos around at launch #muckingin. Refreshing change for wifely duties.

Been watching Sturgeon's husband @PeterMurrell carting boxes of manifestos around at launch #muckingin. Refreshing change for wifely duties.

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In the past week the Conservatives have dusted off two policies closely associated with the 1980s - Right to Buy and privatisation. Given that the Conservatives dominated 1980s politics, it isn't hard to see why they might be tempted to revive some of the Thatcher era's blockbusters.

And yet - so far at least - these two policies (extending the right to buy your home at a discount to housing association tenants and offering the public a discount in shares of Lloyds bank) have failed to provide much of a boost at the polls. Whilst it is too early for much polling on the Lloyds sale, a YouGov poll this week found the public to be against extending Right to Buy.

So, why are policies that proved popular in the 1980s not having the same impact today?

It may partially just be a question of scale.

In 1980, when the original Right to Buy was enacted, around one third of all households lived in a council house. This was a policy that directly impacted a substantial share of voters. By contrast the extension of the scheme to housing associations is far more limited.

Similarly, the sale of Lloyds is comparatively small beer when set against to the sales of British Gas, British Telecom, British Airways and the other big moves of the 1980s. The percentage of UK households holding shares rose from 8% in 1980 to 23% in 1990. Selling Lloyds will have nowhere near as much impact.

Taken together the original Right to Buy and the 1980s privatisations represented a transformation of the UK's political economy. Margaret Thatcher set out to build a "property owning democracy" and she broadly succeeded. Right to Buy 2 and the Lloyds sale will tweak the UK's exiting model rather than transform it.

tweets

Quote Message: Sturgeon refused to say how could vote down Lab budget w/o triggering 2nd elex. I think they're retreating, fearful of Tory chaos attacks

Sturgeon refused to say how could vote down Lab budget w/o triggering 2nd elex. I think they're retreating, fearful of Tory chaos attacks

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Chris has explained today the Parliamentary procedure on how budgets are actually passed with an eye on Trident renewal. I can't help but think that this situation isn't entirely unprecedented.

In 1910 there were two general elections – both of which proved inclusive. In the second of those elections, the results gave the Liberal Party 272 MPs vs the Conservatives on 271. The balance of power was held by the new Labour Party on 42 MPS and the Irish Nationalists on 74. A near tie between the two big parties and a strong showing for a party for a nationalist party that wants to secede from the UK, sounds eerily familiar to modern ears.

The Liberal government was reliant on the Labour Party and the Irish to pass most of its legislation. But when it came to defence, that coalition of votes was less reliable.

There was of course no Trident in 1910, but the nearest equivalent was the Royal Navy. Against a backdrop of a growing German navy, the Conservatives in 1910 had campaigned hard on this issue.

Whilst the Conservatives were almost uniformly in favour of increased naval spending, the Labour Party was broadly pacifist. On the Liberal branches there were those who believed that more defence spending was not a national priority. As one MP put it in a1909 debate: “I do not believe there is a more deadly enemy to social reform in the proceedings of this House than vast expenditure on armaments”.

Given they couldn’t rely on either the entirety of their own party, nor their Labour or Irish allies, how did the Liberal government manage to increase naval spending?

By relying on Conservative votes when needed. It isn't too hard to imagine a similar scenario in the next Parliament. If the SNP vote against Trident renewal then, as Chris hints, do we really expect the Conservatives to sit on their hands and see it defeated?

tweets :

Quote Message: Alex Salmond is not at #snplaunch. SNP press office tell me he is in his constituency. Very strange. #ge2015

Alex Salmond is not at #snplaunch. SNP press office tell me he is in his constituency. Very strange. #ge2015

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Over the weekend, I wrote a blogpost on the perils of the Tory plan to sell Lloyds Banking Group. In short, bank shares are tricksy financial products to own. And if a lot of people with modest savings have invested in the bank, it would make make it politically harder to do the right thing with Lloyds if there is another financial crisis that knocks a hole in its balance sheet.

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Why is the SNP manifesto launch so late?

Labour launched their manifesto a week ago. So why have the SNP held their own launch back? Well, it's pretty simple. Postal votes.

The first postal votes land on doorsteps up and down the country today. There were nearly 800,000 Scottish voters registered for postal votes in last year's referendum - one in five voters.

So the plan is to have Nicola Sturgeon all over the airwaves and front pages just as the first votes of this election are cast.

They know that the more voters they can get out - particularly while the polls are looking good (and they are looking very good for the SNP) - the better. You can't fault the logic.

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This morning, there are several reports that the SNP is pondering whether it could block a Labour minority government from pressing ahead on the renewal of Trident.

The SNP can influence, it can cut deals and, of course, Labour will not want to be seen to ignore Scotland. It is also not in anyone’s interest for there to be relentless negotiation. I assume some sort of deal would be concluded up front.

But if that does not happen or a deal breaks down, the SNP's ability to force concessions on issues such as Trident is perhaps not as strong as it first looks. This morning's newspaper coverage confirms that suspicion to me.

Stewart Hosie, the deputy leader, has said the SNP might “vote against or table amendments to estimates”. It’s worth understanding what that means. Here are a few things worth knowing about the way that our parliament works:

1. Only the Treasury may introduce spending proposals. So backbenchers and the opposition aren’t allowed to lard up bills with pork. MPs may only amend spending bills to cut the amounts pledged in them.
2. The way that public cash gets handed on to departments is through a process known as “the estimates”. That’s what Mr Hosie was talking about. They usually go into legislation through what are known as Supply and Appropriation bills.
3. Erskine May, the constitutional handbook, says these supply bills can be sliced into chunks. And only the Treasury can propose spending bills, so opposition MPs cannot re-amalgamate them. Oppositions can only cut the totals proposed or vote them down.

So how about this?

A Labour Treasury could split the supply bill in two and defence spending could be hived into its own standalone slug of legislation, which would be voted on first. This sort of division would allow the SNP to abstain on Trident then pass the rest.

What if the SNP sought to play hardball?

In the event that the SNP sought to vote Trident spending down, the nationalists would have to contend with two problems. First, they would not have the votes to propose some specific bar on funding for Trident from the bill: there is a strong pro-Trident majority in parliament.

So their best hope would be shooting down the whole defence budget bill. But winning that vote would mean they would be open to the accusation that they are cutting off the budget lines that pays for salaries for soldiers and pensions for veterans. Would that be politically wise?

Second, even then, the SNP would need to get the Tories to vote with them. That seems unlikely. The Conservatives are unionists and we are talking about defence. It is an area of policy where lots of Tories feel that they have a particular historic responsibility and strength.

The Tories could perhaps justify voting down a defence supply bill on procedural grounds. But it is worth remembering that oppositions can cause mischief by supporting governments: David Cameron made hay by backing the 2006 Education Act, for example.

If that defence bill passed, would the SNP then be brave enough to start trying to vote down the next supply bill? For the NHS? For welfare? For the Scottish block grant? Would they vote down Budget resolutions? Or the Finance Bill? And, again, would they get the Tories to vote with them on this effort?

If the SNP could force concessions out of Labour through such a ferocious approach, would an alliance with the Conservatives against Labour shore up support in Scotland?

And, at that point, they would be very near to bringing down the whole government. Labour could make these bills into confidence issues. At that point, the SNP's problem is that their best weapon would be to bring down the whole government - too devastating a weapon to fire on almost any single topic.

How appropriate that this argument should revolve around nuclear weapons.

p.s. Colin Talbot, a professor at Manchester, has reached a similar conclusion via slightly different reasoning.