Arithmetically, not very much.
One scenario for the 2015 election sees the Tories managing to hold pretty much all the seats they won in 2010, while losing a few to Labour but gaining some from the Liberal Democrats in return.
Do they decide, unlike last time, to try and form a minority government? The main reason the Conservatives cited in 2010 for going into coalition with the Lib Dems – the “economic crisis” – no longer applies. Were the party to once again fall short of the number of seats needed for an absolute majority (326) but only by two dozen or so, a minority government would undoubtedly be an option.
Remember that the figure of 326 includes Sinn Fein, whose five MPs refuse to take their seats at Westminster. Subtract them from the equation and the number of MPs a party needs for a working majority falls to 323.
Let’s say the Tories profit from a collapse in the Lib Dem vote not merely in the battleground areas of the south and south-west, but right across the UK. They could win as many as 12 seats from their current coalition partners. A swing of around 3.5% would see the following seats change hands: Solihull, Dorset Mid & Poole North, Wells, St Austell & Newquay, Sutton & Cheam, St Ives, Somerton & Frome, Chippenham, Berwick-upon-Tweed, Cornwall North, Argyll & Bute and West Aberdeenshire & Kincardine. The biggest majority the Tories would have to overturn is 3,684.
Even if they lost 10 or so seats to Labour, so long as the Tories picked up a dozen from the Lib Dems they could end up roughly where they are now: 306 seats. That’s 20 short of an overall majority, as I noted the other day, but only 17 short of a working majority.
Imagine the scene: the Lib Dems are in crisis having lost around half of their MPs, with Labour, the Tories and even the SNP making gains at their expense. Nick Clegg’s leadership is being openly challenged, but party members are at odds over whether to form another coalition with the Tories, a new one with Labour, or avoid coalition government entirely.
Labour is reeling from the shock of having performed so far below expectations, not even managing to overtake the Tories in number of seats.
Meanwhile the Tories, being the largest party, are invited by the Queen to try and form a government.
David Cameron senses the only way he can remain prime minister is to go for a minority administration and dare a broken Lib Dem party and an exhausted Labour to vote him down in a confidence motion.
His gamble pays off and a Tory minority government takes office. Six months later, Cameron goes to the country again, urging voters to give him and his party enough support to form a stable, majority government and bring this “constitutional crisis” to an end. He makes an EU referendum the centre-piece of his campaign.
Come polling day, enough Ukip voters swing back to the Tories to hand Cameron a majority large enough for a full five-year term. He returns to parliament triumphant, to face the new leaders of both Labour and the Liberal Democrats…