The first poll on the Eastleigh by-election, courtesy of Lord Ashcroft , suggests that the contest will be as tight as expected. The Conservatives are in the lead on 34 per cent, three points ahead of the Lib Dems, who have held the seat since 1994 (another by-election). But when all responses are included, rather than those certain to vote, the positions are reversed, with the Lib Dems three points ahead of the Tories (32-29). The challenge for Clegg's party, which holds all  36 council seats in the constituency, will be getting out its vote.
Labour is in third place on 19 per cent, an increase of nine points since the general election, but far behind the Lib Dems and the Tories. On last night's edition of This Week, Alan Johnson bluntly declared : "Labour aren't going to win."
Among other things, then, Eastleigh is a reminder that tactical voting will be a major issue in 2015. Indeed, if the Conservatives win on 28 February, it will become an issue immediately. The Tories are in second place in 38 of the Lib Dems' 57 seats and half of those  on its target list are held by Clegg's party. If Labour is to prevent the Tories from decapitating scores of Lib Dems, it will need to consider whether to advise its supporters to cast tactical votes. In 2010, Ed Balls and Peter Hain both argued  that Labour supporters should consider lending their votes to the Lib Dems in seats where the party couldn't win. But after five years of Clegg and co. acting as the Tories' "accomplices", it is doubtful whether many Labour figures will repeat this call.
The biggest electoral headache for the Conservatives remains that any collapse in the Lib Dem vote will work to Labour's advantage in Tory-Labour marginals, as was shown in the Corby by-election . If this patten is repeated at the general election, the Tories stand to lose dozens of seats - there are 37 Con-Lab marginals where the third place Lib Dem vote is more than twice the margin of victory.
If they are to stand any chance of winning a majority at the next election or even remaining the largest single party, the Tories need to hope for a partial Lib Dem recovery.