In the absence of a significant change in the polls, the Tories will not be able to survive in power after the election. The SNP's pledge to vote down any Conservative-led government means that David Cameron will lack the support he needs to remain prime minister if, as seems likely, the nationalists hold the balance of power (as I noted in my column last week). Even if the Tories finish ahead of Labour on votes and seats, they will still struggle to assemble the 323 MPs they need to govern (323 being the number required for a majority in the absence of Sinn Fein). Our sister site May2015 has done the essential number crunching.

This creates the possibility that Labour could enter power even if it finishes second. Jonathan Freedland is one of those who has raised doubts over whether Ed Miliband could become prime minister under this scenario. He wrote in the Guardian: "If the Tories emerge as the largest single party, they and their cheerleaders will claim at least a partial victory no matter how distant they are from a Commons majority. Their aim will be to make it hard for Miliband to form a government – not numerically hard, but politically hard. The Tory script will say things like: 'These are the rules. If you win a test match by one run, it’s still a win. And Labour lost.'" Freedland concluded: "To head off the coming battle over legitimacy, Labour will have to be the largest single party as well. For all the complexity of our current politics, the truth is actually quite simple: the only way to be sure of getting a Labour government is to vote Labour."

It would certainly be uncomfortable for Miliband to become PM in these circumstances. As Jim Murphy is fond of pointing out, not since 1924 (when Ramsay MacDonald became the first Labour PM) has the party which finished second on seats taken office. But it should not be regarded as impossible. A government will have to be formed eventually and if Miliband has the Commons support he needs he will lead it.

Outside of the UK, there are more recent examples of second-placed parties entering office. As Andrew Adonis, one of those leading Labour's preparations for government, wrote in his 2013 book 5 Days in May: The Coalition and Beyond: "Willy Brandt's great government was a coalition between the second-placed SDP and the liberal FDP; the current Swedish Conservative government is led by the second largest party, a long way behind the Social Democrats in seats and votes." If Labour does finish second, but is the only party able to form a government, expect these precedents to be widely cited.

It would be a shock to the UK system for a second-placed party to govern (just as it was for a coalition to be formed). But it is one that would be manageable. If it finally leads to the abandonment of our antiquated first-past-the-post system, all the better. For now, these are the rules under which general elections are fought. After that, our leaders are at the mercy of the Commons.