Rochester by-election analysis: David Cameron acted in haste – and is now repenting at leisure

Tory strategists let it be known that Rochester and Strood was a seat the party could win

Ukip leader Nigel Farage arrives at the Rochester by-election count (Getty Images)

It was a rash decision - made to avert an immediate crisis - but one that David Cameron is now regretting at his leisure.

In the aftermath of Mark Reckless's defection to Ukip on the eve of Conservative Party conference in Birmingham Tory strategists let it be known that Rochester and Strood was a seat the party could win.

The constituency, they said, with its 10,000 Tory majority at the last election, was not like Clacton and Ukip's anti-immigration message would not resonate to the same extent.

What's more, they added, Reckless did not have the same personal following as Douglas Carswell and could be beaten by the right candidate with the right message.

Today that sounds like foolish hubris.

The first is that Ukip is a scourge on the electoral base of all three main parties. Ukip's votes in Rochester did not just come from the Tories they came from dissatisfied Labour and Lib Dem voters as well. In fact the resonating message from talking to voters on the door step was all the main parties are just as bad as each other so we're going to give this new lot a try.

But that is not, however, the same as saying that Ukip on the verge of a nationwide electoral breakthrough and could unseat the Lib Dems as the third party of British politics.

The first-past-the-post system means that Ukip are very unlikely to win more than four or five seats next May. In fact, its small majority in Rochester suggests that Mr Reckless may well be booted out from his seat in just a few months time.

But the impact they will have - and the reason why they are getting so much publicity - is the effect they will have on the 100 or so marginal seats that will determine the next election.

If they can succeed in taking 10-15 per cent of the vote in those seats then they could have a disproportionate impact on the overall result.

At the moment polling suggests that Ukip will take more of the Tory vote than that of Labour. But with Labour facing a potent electoral challenge from the SNP in former Scottish heartlands, both parties could lose seats they might otherwise have expected to win comfortably.

In short, we're on the verge of what's likely to be the most unpredictable General Election in living memory.