What should worry David Cameron more, Douglas Carswell’s defection to Ukip or reports that as many as 100 Tory MPs could go into the general election pledging to leave the European Union? The former is certainly more dramatic and promises plenty of humiliation over the next few months. But the latter could show the Prime Minister that he doesn’t have as much time on European reform as he would like, and that he is still not trusted by a large contingent of his party.

It is one thing for Better Off Out members such as Mark Reckless to pledge to campaign to leave, no matter what reforms David Cameron manages to secure. But it is much more serious if other MPs decide that there is little point in holding their breath for whatever Cameron can produce by 2017, either because they think voters have already made up their minds and will only reward an Outist MP at the ballot box in 2015, or because they don’t think Cameron will produce anything significant. Carswell cited this fear when he announced that he was leaving the party, even though he had made his mind up already that Britain was better off out. Other MPs have not yet decided how to campaign – and Jacob Rees-Mogg last night told Westminster Hour that ‘if the Prime Minister hasn’t by the election set out very clearly his terms for renegotiation and the circumstances under which he would call for a ‘No’ vote’, he would be very tempted to put a statement in his election literature saying he would campaign to leave.

The Prime Minister has so far seemed relaxed about the renegotiation efforts he must make before the election. But here is the ultimatum from eurosceptics that became inevitable as soon as Carswell defected: we will cause trouble for you unless you tell us what your shopping list is. And this is the last thing that the Prime Minister wants to do, not least because his shopping list will not satisfy as many eurosceptics as he needs. But it clear that he cannot put this off until after the election: a referendum pledge on its own is no longer enough.

Tags: Conservatism, Douglas Carswell, eurosceptics, UK politics