Where is this burning point of principle that drove Douglas Carswell into the arms of Ukip? I’ve read lots about his defection, and I’m still none the wiser. We’re told that he was talking to Farage for almost a year, which would have overlapped with the time he told me that the Tories need to unite behind Cameron because he was the only one promising an in-out referendum. What has changed?

Carswell says that Cameron is not serious about Europe. The Prime Minister has become the only leader in the continent to promise an in-out referendum. I’m not sure how much more serious one can be. Should he lay out, now, what he wants in a renegotiation? Of course not – it would be rendered rapidly out of date. The referendum would be three years away, and Europe is changing all the time. Back in January, Carswell told me that he understood this. No longer.

So what’s going on? I can recognise the argument Charles Moore makes: that the ‘modernising’ project now stands exposed as an abject failure which hollowed out, rather than strengthened, the Conservative Party (its membership has halved under Cameron). I can see the critique of the Tories from the right, summed up by Peter Hitchens today. We can also see the old Tory wars, where the left and the right of the party want to fight each other more than Labour. Even Matthew Parris is saying that it’s time to draw the battle lines. You can add to this perhaps the single greatest weakness in the Cameron project: a lack of a political message (the absence of a political operation in No 10 is a symptom, rather than a cause, of this malady).

Yes, there are many reasons to despair at the Cameron project. But next June, we’ll have one of two options: David Cameron in No 10, and a referendum in 2017 or Ed Miliband and no referendum. In trying to inflict damage on the Conservative Party now, a few months before an election, Carswell makes the second outcome more likely. As James Forsyth argues today, if Carswell wins then the bones will never heal and Labour will have an inherent electoral advantage akin to the one the Tories enjoyed in the 1980s. Carswell will know this. There is obviously something he wants more than a referendum.

Carswell churns out practical blueprints for government – he understandably wants to see them adopted. Some were taken on by the Tories, but never with any attribution (or thanks) to him.  I suspect he thinks more of his ideas will be adopted by Ukip than by the Conservatives. After all, Ukip doesn’t care too much about policy – Nigel Farage openly trashed his party’s last manifesto (‘I didn’t read it – it was drivel,’ he said). So Farage could be the face and Carswell the brains. He may hope that his pamphlets will, in time, become Ukip party policy. And become more forceful in the wrapping of a political party than a digital e-book.

Much as Carswell may wish it otherwise, the British political system gives voters two choices – and next May, that choice will be reform with the Conservatives, or a Brownite restoration with Miliband. Parties evolve, as the Tories have under Cameron: the radical welfare reform agenda, for example, was not on the menu before the 2010 election. Had Carswell stayed he could have helped the Tories evolve. Now, he has ended up helping Labour’s plan to evolve – from opposition to government. It really is that simple.

Tags: David Cameron, Douglas Carswell, Nigel Farage, UK politics, UKIP