On 28 August 2014, Carswell announced his defection to UKIP. Although not required to seek re-election following a change of party allegiance, Carswell triggered a by-election, held on 9 October 2014, in which he was elected as the UKIP candidate. He became the first elected UKIP MP. Carswell retained the seat for UKIP at the 2015 general election, seeing his majority cut by roughly three-quarters, with an 11% swing to the Conservatives. Carswell then became UKIP's sole MP in the House of Commons, as Mark Reckless, a fellow Conservative defector, lost his seat.