Ukip’s future is in the balance. Yes, it has doubled its representation in the House of Commons. Yes, it has survived a far more ferocious onslaught in Rochester and Strood than it faced in Clacton just six weeks ago. Yes, David Cameron visited the Kent constituency five times and Ukip’s latest MP Mark Reckless still emerged victorious.

And yet… Ukip’s majority was less than Nigel Farage must have hoped—and Cameron feared. Last month Douglas Carswell held Clacton by 12,000 votes; Reckless won by fewer than 3,000. Carswell added seven percentage points to his general election vote share (up from 53 per cent to 60 per cent); Reckless lost seven points (down from 49 per cent to 42 per cent).

On these figures, Reckless would have to defy history to hold Rochester next May. Normally when insurgent parties (such as the Liberal Democrats and SNP in days gone by) win by-elections, they benefit from the protest votes of people who know that they can send a message without deposing a Prime Minister. Such votes often return “home” in the following general election. By-election victors need the cushion of a big victory to survive this loss of support and retain their seat when Britain’s government is at stake.

Reckless’s cushion is not big enough. A swing back to the Conservatives next May of just 3.7 per cent would lose him his seat. On its own, the fate of a single MP out of 650 would be of no great importance. The point is that other Tory MPs can do—indeed will already have done—the same sums. Those who have been contemplating defection may decide that his win was not big enough for them to follow his example and switch parties.

That could make life a bit tricky for Ukip, even as they celebrate today’s victory. A defector every few weeks between now and next April would have kept the party in the news and sustained its momentum. Now Farage may have to find other ways to keep in the public eye. Given his record, few will be foolish enough to bet against him doing this. And, of course, he could still win over Tory peers, and MPs who are standing down, for they won’t be facing the electorate next May.

Even so, the Ukip bandwagon no longer looks unstoppable. I reckon that it’s a 50-50 call whether the party resumes its upward surge after a pause for breath or finds that the going from now on becomes a great deal harder.