His resignation is not a total surprise as it follows a long period as a critic of the Conservative Party's direction and of David Cameron in particular. Carswell's criticisms are not limited to the Conservatives and not limited to Europe - he has developed a critique of British politics generally - but Europe is among his key complaints.
In his resignation speech Carswell questioned David Cameron's commitment to EU reform accusing him of aiming to do the bear minimum necessary in order to secure an 'in' vote, while Carswell's view of a satisfactory renegotiation seems more akin to associate membership. He says this is a classic example the political class not being straight with the electorate and his reason to quit. The Conservative leadership for their part will feel aggrieved that having set out a 2017 referendum on EU membership they are rewarded with Carswell's 'ingratitude' - recriminations will run and run.
Carswell prides himself on having a large following locally and Clacton itself, it has been argued, is the "number one most demographically favourable seat in the country for UKIP" according to Goodwin and Ford's Revolt on the Right. He therefore has a fighting chance of winning the seat. So what could happen?
Firstly, it is unclear whether the Conservatives will allow the by election to go ahead - they have to approve the writ being moved and could argue that with an election already scheduled the people of Clacton can wait until May 2015. However, if it is called, the stakes could not be higher.
For the Conservatives to hope to win the 2015 general election they need to minimise the UKIP vote. The best way to do this is the classic 'squeeze'. In a first past the post election they will say there is no point voting for a third placed party - i.e UKIP. This will be very effective. However Carswell's by-election could change voter's calculations - and set a hugely important electoral precedent one way or another:
- If Carswell wins, UKIP can then tell voters everywhere that voting UKIP gives a genuine change of producing an MP. This could be catastrophic for the Conservatives and may deprive them of a number of seats, to other parties mostly, but conceivably to UKIP as well.
- If Carswell loses, the result will be equally disastrous for UKIP. The Conservatives can use it to show that even in one of UKIP's best constituencies with one of their biggest names they can not win a seat - so why waste your vote in May 2015 will be the refrain.
In any case, a Labour victory would most likely mean that the In/Out EU referendum Cameron has promised won't happen, as Ed Miliband looks determined to stick to his promise not to offer a straight vote. It would be a tremendous irony if Carswell's defection - Carswell, remember, having campaigned tirelessly for an EU referendum and more direct democracy - would in the end deny the UK public an EU referendum and more direct democracy.