What is going on? PMQs was oddly revealing, but not for the despatch box name-calling, nor for the debate debates. Mr Cameron said Labour were “not trying to win, just trying to crawl through the gates of Downing Street on the coat-tails of the SNP… crawl to power in Alex Salmond’s pocket”. He brandished this Scottish Labour leaflet:

“we need to stop the Tories being the largest party. That’s the only way to stop a Tory Government”. The leaflet is a rather public lowering of expectations, but also a reflection of a specific debate in Scotland. The SNP have rather cunningly prosecuted the argument that Labour and SNP seats are interchangeable and will not be used to support a Tory Government. So the third of Labour voters who voted Yes at the referendum, can safely vote SNP (as they may do at Holyrood elections anyway) without fear of prolonging David Cameron’s tenure at Downing Street. It has been an effective political manoeuvre. Labour has tried to counter by deploying the argument that less Labour MPs increases the chances of the Conservatives remaining the largest party in any hung parliament, and the leaflet is a part of that. In turn, the SNP Leader has suggested to me that the SNP could support Labour even if they are the second largest party http://news.sky.com/story/1425670/snp-could-make-pm-of-second-place-miliband.

Of course, there is something rather telling about the PM’s use of this argument. It is only really a relevant issue if the Conservatives fail to win a majority. Grant Shapps and team at CCHQ talk of a majority being just 11,000 votes away. But the PM is already playing the post-election minority government legitimacy game. He still expects to be leading the largest party, as he does now. But the SNP will be winning Labour seats on the basis of those seats never being used to support a Conservative. Small wonder that the PM is trying to bounce Labour into ruling out a deal with the SNP. The PM wants to punt those seats likely to be lost by Labour to the SNP firmly away from Labour for post-election dealmaking purposes. In that situation, it would be more the Conservatives heading back to Downing Street “on the coat tails of the SNP” success, if not necessarily being in their pocket.

Labour’s instincts are however that it can govern without a formal deal with the SNP. The SNP would be obliged to go along with its initiatives because it would have no mandate to topple a minority Labour Government in a confidence motion or at a Budget and return the Conservatives, particularly given next year’s Holyrood elections.

All of which means that the race to become largest party in a hung parliament is a very real one. For Labour the Maths is this: can it make 23 direct gains from the Conservatives? Tot that up and its is 22,000 extra votes in blue/red marginals over Gordon Brown’s efforts from 2010, and number 23 is Mary Macleod’s constituency of Brentford and Isleworth. Labour will then need another Conservative seat for every two it loses to the SNP. Throw in a dozen or so Lib Dem seats too. Equally, looked at in those terms, and the Conservative challenge is to hold its ground in just two dozen Labour facing seats, and then let the SNP feast on Labour in Scotland, whilst enjoying Labour difficulties in trying to hold Scotland and appeal to the English centre ground at the same time.

We will be able to see some of this in the campaign, in the choice of Leader visits, campaign spending etc. Either way, a secondary “winning” post for largest party of about 290, not 326, is worth remembering. It should not have a particular constitutional significance, as Lord O’Donnell told me in January, http://news.sky.com/story/1402051/election-winner-may-not-get-keys-to-no-10 , but both main sides seem to be implying in their actions, if not their public words, that this is their more modest secondary goal.