One reason why by-elections are a poor predictor of general election results is that voters behave very differently in the former to the latter. People can vote for the party of their choice secure in the knowledge that only the identity of their MP, not the government of the country, will change.
With this in mind, a new poll from the ever-prolific Lord Ashcroft (recently profiled by Andrew Gimson for the NS) offers useful evidence of how the result in Eastleigh could change in 2015. It shows that just 43 per cent of Lib Dem supporters in the constituency would vote for the party at the general election, with 13 per cent likely to defect to Labour, seven per cent to the Tories and a third undecided. Similarly, only 43 per cent of current UKIP voters expect to stick with the party in 2015, with 10 per cent planning to vote for the Conservatives.
Encouragingly for the Tories, 73 per cent of their supporters expect to vote for the party in the general election, with 23 per cent undecided. If we strip out the don't knows, the Conservatives enjoy a nine-point lead over the Lib Dems (33-24), with Labour in third place on 24 per cent and UKIP in fourth on 16 per cent.
Is this strong evidence that the Tories will win Eastleigh in 2015? No, the high number of Lib Dem don't knows (34 per cent), at least half of whom are likely to return to the fold, as well as the small sample size (760) means we should be wary of drawing any conclusions. But on an otherwise dark day for the Tories, the Ashcroft poll gives the party a glimmer of hope.