General Election 2015: how many seats will the Lib Dems lose?

Whither the Liberal Democrats? Since 2010 they have lost two-thirds of their support. They are bound to lose a fair number of seats in May. But how many? And, will they still be in a position to affect who governs Britain afterwards?

They currently enjoy 7 per cent support in Britain as a whole, with just 4 per cent in Scotland. On conventional assumptions about uniform swing—though calculating Scotland separately from England and Wales—the party would lose 41 seats, and end up with just 16. In one fell swoop, the party's gains in the past four general elections would be wiped out.

They won't do that badly. They can count on two factors to save some of those 41 seats. What is less certain is quite how big those factors will be.

Factor 1. The Lib Dems usually gain support nationally during election campaigns. They benefit from the exposure the main broadcasters are obliged to give them under the current rules. This should help them a bit; however, in the past, this boost has been augmented by the party's status as a protest party unsullied by the compromises and disappointments of government. They won't have that extra advantage this time.

Factor 2. Lib Dem MPs often build up a personal following. Voters like them and think they do a good job serving their local communities—the Energy and Climate Change Secretary Ed Davey holds a sizeable majority Kingston, as does Julian Huppert in Cambridge. They can't expect to insulate themselves entirely from the contempt with which voters hold the party nationally, but they might lose less support than Lib Dem candidates in the rest of Britain.

However this incumbency bonus won't apply to the 11 Lib Dem MPs who are standing down. Ten look to me to be likely losses. The 11th is Bath, where the Liberal Democrat Chief Whip Don Foster, who has held the seat since 1992, has bequeathed his successor an 11,883 majority. It's touch and go whether the Tories can recapture what was once Chris Patten's constituency.

My guess—and it can be little more at this stage—is that the Lib Dems will lose 25-30 seats, in roughly equal numbers to Conservative, Labour and the SNP. But if the party does worse than I expect, the extra losses are likely to be more to the Tories.

This is not just about apparently esoteric numbers. Suppose the Tories are the biggest party in the House of Commons after 7th May with 295 seats, while the Lib Dems hold 30. This could lead to a new Conservative-Lib Dem arrangement of some kind—whether a formal coalition or something looser. Cameron could find it hard to win a vote on the Queen's Speech otherwise.

On the other hand if the Tories capture an extra ten seats, pushing the Lib Dems down to 20 while David Cameron's ranks swell to 305, then Nick Clegg will have little power, even if he is one of the 20 survivors. The Conservatives could enlist the support of the eight or nine Democratic Unionists in Ulster, and remain in power for at least a while at the head of a minority government.