Nigel Farage is crowing about the Conservatives "splitting the UKIP vote", while the Lib Dems brag, "if the Tories can't beat us now, when can they?" Last night was not a good one [2] for David Cameron. One should always be wary of extrapolating from by-elections, which are a famously poor predictor of general elections, but with this proviso, there are two reasons why Eastleigh bodes ill for the Tories' prospects of victory.

The first is that it suggests the Lib Dems will benefit from an incumbency factor in 2015. In those seats where the party is well organised and where it can appeal for tactical votes from Labour supporters, it can still win. This is largely a problem for the Tories, who are in second place in 38 of the Lib Dems' 57 seats, and whose hopes of a majority rest on taking as many as 20 seats off Clegg's party. Eastleigh suggests it will be much harder to dislodge "the yellow bastards" (as the Tories affectionately refer to their coalition partners) than they hoped.

To add to the Tories' woes, the likely collapse in the Lib Dem vote elsewhere will work to Labour's advantage in Conservative-Labour marginals, as Corby [3]demonstrated. 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.

The second reason why Eastleigh is so troubling for the Tories is that it shows the UKIP problem hasn't gone away. Those who predicted that Cameron's promise of an in/out EU referendum would do little to dilute the appeal of the "none of the above" party were right to do so. UKIP may yet fail to win a single seat at the next general election but it will almost certainly improve on the 3.1 per cent of the vote it attracted in 2010. Again, this is primarily a problem for the Tories, whose voters still account for a greater share of UKIP support than the Lib Dems' or Labour's. At the last election, there were 21 seats in which the UKIP vote exceeded the Labour majority. The prospect of a surge in this number is the main reason why some Conservatives are already urging Cameron to look again [4] at the possibility of a Tory-UKIP pact, an option flatly dismissed by Michael Gove on the Today programme this morning.

Weary of the shackles of coalition, Tory MPs are desperate for evidence that they can achieve the majority that eluded them in 2010. But the resilience of the Lib Dems and the continuing division of the right means the Tories' chances of outright victory are looking slimmer than ever today.