The Rochester and Strood result may make Conservative MPs ready to defect think twice

There is perhaps no clearer indication of just how far Ukip has come in this parliament than the fact that its second by-election success came as little surprise.

The Conservatives might have threatened to throw the kitchen sink at denying Mark Reckless the pleasure of retaining his seat under his new colours, but the polls had long since suggested that he was going to succeed. In the event his former colleagues were left clutching at the straw that, at just over seven percentage points, the Ukip majority was rather less than the polls had anticipated.

But even if it was expected, the victory still matters.

When Ukip topped the poll in May’s European elections, both Conservative and Labour politicians promised us that the Ukip bubble would burst once voters focused on the fact that in next year’s general election the only real choice is between David Cameron and Ed Miliband.

Yet six months on, the bubble is as big as ever. Ukip’s initial victory in Clacton last month boosted its support to an all-time high of 16 per cent in The Independent’s monthly poll of polls. Now its second success will give further credibility to the party’s claims that it represents a serious alternative next May. It now looks even less likely that its support will fall back to the 3 per cent it secured in 2010.

Given the polls continue to suggest that Ukip is drawing support more heavily from the Conservatives than from either of its main rivals, that is bound to make it harder for Mr Cameron’s party to win the overall majority it craves.

Yet the straw that the Conservatives have been left clutching matters too. Douglas Carswell’s success in winning no less than 60 per cent of the vote in Clacton indicated that he has an excellent chance of being re-elected next May, even though voters are more willing to vote Ukip in a by-election than they will be in next May’s general election.

Mr Reckless’s more modest 42 per cent suggests he will have a fight on his hands when he attempts to retain his seat. Any Tory MP who might have been contemplating becoming the third defector may well now be wondering whether they really would have a better chance of defending their seat if they switched.

Keeping the air in the Ukip bubble could well prove a more formidable challenge in the next vital six months.

John Curtice is Professor of Politics, Strathclyde University