December’s general election was disastrous for the British left. In 2017, Corbyn’s Labour Party surprised the pundit class by scoring a 40 percent vote share, ten points up from 2015, and enough to deprive the Conservatives of a majority. But in 2019, the party shed 2.5 million voters from that high point, gifting Boris Johnson a comfortable majority, with Labour falling to its lowest number of seats since 1935. As the battle commences over what the party does next, how we explain this dramatic transformation in fortunes will be crucial to determining the future.

The most obvious explanation for defeat focuses on Labour’s Brexit policy. In 2017, Labour had pledged to respect the result of the Brexit referendum, promising only to negotiate a closer relationship to the European Union (EU) than that offered by the Conservatives. By 2019, however, the party had committed to a second referendum on Brexit, with the option to remain in the EU. Those who warned that this decision would cost votes in Labour’s traditional heartlands were proven correct when the party bled seats in the Midlands and North of England, some of them held by Labour for over a century.

With fifty-two of the fifty-four constituencies Labour lost to the Tories having voted Leave, this account is credible. It also brings with it some comfort for the Left, as the key backers of a second referendum were Labour’s centrists. Corbyn’s biggest error, in this narrative, was his accommodation of liberals, who applied pressure within and outside the party to force the adoption of a policy that would become a millstone round Labour’s neck.