One consequence of the Tories moving ahead of Labour for the first time since March 2012 is that MPs of all parties are beginning to discuss the possibility that the Conservatives could win the most votes in 2015 while Labour wins the most seats. Such an outcome is made possible by the first-past-the-post system. While the Tories pile up wasted votes in high turnout areas, Labour efficiently pockets city constituencies where fewer take part.

On a uniform swing, Lord Ashcroft's poll (which puts the Tories on 34 per cent and Labour on 32 per cent) would leave Labour with 14 more seats (307 to 293) than the Conservatives. David Cameron requires a lead of around four points before his party moves ahead.

Were Labour to win more seats with fewer votes (as last happened in February 1974 and in reverse in 1951), the Tories would have no grounds for complaint. They have resisted every attempt to reform the system, most recently in the case of the AV referendum. (The failed boundary changes would have reduced but far from eliminated Labour's advantage.) But the outcome would be widely seen as perverse and undemocratic. Alongside this, it is possible that Ukip (a pro-reform party) could receive more votes than the Lib Dems but win no seats.

All of this would have the effect of reviving the debate over electoral reform, which has laid dormant since the defeat of AV in 2011. The Lib Dems and Ukip would be in a strong position to push for a referendum on proportional representation (although I'm told that PR for local government is a more likely coalition red line) and the flaws of a system designed for an era when the two main parties won 96 per cent of the vote between them will have been exposed as never before.