Douglas Carswell's decision to trigger a by-election by defecting to Ukip is a principled one - but it's a calculated one, too.
The Clacton MP, who single-handedly handed David Cameron the prime minister's biggest setback of the summer, has come to believe his own brand of personalised politics will trump the party machine set to descend on his constituency in the coming weeks.
Carswell, after all, is asking a lot. He will have to do better than the only other Ukip MP in the party's history, Bob Spink, who defected in 2008 but was roundly defeated in 2010. At the same general election Carswell won 22,867 votes for the Conservatives, handing him a 12,000-plus majority over second-placed Labour. The question now is how much of that advantage belongs to him - and how much of it is about loyalty to the Tories.
Seventeen months ago he gave Politics.co.uk his take on exactly that question. As has now become clear, Carswell had been thinking about jumping ship for some time. Perhaps, back then, he was weighing up the feasibility of his political career surviving this very risky move. Certainly the logic he will have used to make his big 2014 decision is based on the same thinking.
There is a "huge slice of the electorate", Carswell said, which doesn't care about party at all. In Clacton, he estimated a blank-faced Conservative candidate wouldn't be able to attract more than a third of the vote. In 2010 Carswell won 53%. Where did the extra 20% come from? The answer is the same one which explains Ukip's growing success: "The anti-politics vote."
Carswell's war is with the cliques of Westminster, not the party system itself. It's a critical decision which explains why a man so desperate to escape from the grasp of the Conservative whips could choose to pledge his allegiance to another party leader. In Carswell's eyes, Ukip stands for independence from the dominant parties in parliament as much as it does from the EU.
No wonder it was attractive. Ever since first being elected as the MP for Harwich in 2005, the former head of the Conservative party policy unit has been depressed by what he's seen. Rather than finding noble-minded individuals seeking to better their country, he says, all he found was "a lot of whips' pigmies squabbling amongst each other for baubles".
The logical next step would have been to walk away for good. But amidst the public anger surrounding the expenses scandal, Carswell found a new purpose: a radical champion for reform. It was he who masterminded the campaign to force Speaker Michael Martin to step aside. The spirit of reform which infused Westminster persuaded Carswell to stay on.
He was, again, disappointed. In particular his efforts to persuade ministers to adopt a decent recall mechanism for misbehaving MPs faltered in the face of ministerial conservatism. Carswell took solace in embracing the internet, using it to build support for himself rather than his party. He believes an online presence allows MPs to "repersonalise" politics. Not all take it - but Carswell has, arguably more than any other.
A big part of his appeal are his principles - not something which can be said about every politician. His commitment to participatory democracy - letting the people have their say - meant it was inevitable that he would accompany his defection with an opportunity for voters to reject the idea, and him. The coming by-election is a gamble, but a calculated one. While the Tories will spend £100,000 against him, he has Ukip's anti-establishment appeal as well as his own following to rely on. The Farage factor shouldn't be underestimated. After all, in this year's European elections the Conservatives barely managed to notch up half the amount of votes won by Ukip.
"I don't want to sound too much like a politician, but like all good politicians I've been on a journey," Carswell told Politics.co.uk last year. He has come a long way since backing Cameron as his party's leader in 2005. While damning him with faint praise as "fun" and a "likeable person" today, he has delivered the man he once backed to be prime minister a hammer blow. The schism on the right of British politics just got a lot wider - exactly, have no doubt, the effect Carswell will have hoped to achieve.