In the first of 1974’s two general elections, Labour won more seats, the Conservatives more votes. The aftermath saw the growth of a group called Conservative Action for Electoral Reform (CAER).

The same outcome is possible in May. As YouGov’s Anthony Wells noted in his end-of-2014 review, Labour’s poll lead keeps falling. But the distribution of the vote gives it a big bang for its voting buck.

First past the post offers diminishing returns to the Tories. None the less, most of them stood foursquare against changing the voting system in 2011. Would that united front crack in the circumstance I sketch?

A case against first past the post is that it works best when two big parties have an electoral monopoly (more or less). One or the other will therefore win over half the vote in most seats. This legitimacy arguably breaks down if you have, say, a four party system in which none can glean much over a third of the vote – which is more or less where we are now.

A case for it is that there’s no necessary link between winning half the vote and getting a legitimate result. First past the post has been used in this country when we had multi-member constituencies, when there was a mass of independents (in the nineteenth century), and when three main parties fought for votes (during the 1920s). There is justice in the candidate with the most votes winning.

Others are also thinking ahead about the consequences of a hung Parliament. Sir Alan Duncan today becomes the most senior Conservative to call for the scrapping of the Fixed Terms Parliament Act.

Elsewhere on this site, Peter Franklin raises the possibility of the SNP giving a minority Labour Government the confidence and the Conservatives giving it the supply (on spending control, anyway).

So what happened to CAER? In short, Margaret Thatcher’s election wins under first past the post killed it off. She and John Major were able to get the Tories over the 40 per cent mark four times in a row.

I’m not in favour of switching systems – and nor, much more importantly, are voters. But the longer no party can gain more than about a third of a vote, the louder the debate will become.