While both parties together received nearly 82 percent of the votes, they are politically further apart now than at almost any time since 1983, when Labour was also more openly socialist. Britain has simply become much more fiercely divided ideologically, with the cross-party consensus of pro-European neo-liberalism in tatters, along with the now derided “third way” of Mr. Blair, the last Labour leader to win an election, let alone three in a row.

Mr. Corbyn has pulled the party back to the harder left, promising more state ownership and economic intervention. His passionate campaign consolidated his leadership and the dominance of the “Corbynistas,” although many Labour legislators fear that a hard-left party cannot win enough votes across the country to regain power.

But Mr. Corbyn’s manifesto was intended to respond to popular dissatisfaction with seven years of Conservative austerity and cuts to social welfare benefits. It made sweeping commitments to more spending on everything from the health service to the police, and promised young people free tuition, a higher minimum wage and another four holidays, while advocating renationalizing the railways and utilities.

It would all be paid for by increased borrowing and sharply higher taxes on corporations and those paid more than $104,000 a year. Taxation would have been the highest ever in peacetime Britain, according to the independent Institute for Fiscal Studies.

With the British economy already heading into the doldrums, in part because of looming Brexit costs, low productivity and a national debt approaching 90 percent of gross domestic product, the Labour platform frightened the middle class and businesspeople and was, to some degree, a fantasy, given that even Labour leaders did not expect to win the election.

Still, despite Labour’s better performance and its success in denying Mrs. May a majority, the party has lost its third general election in a row. With its strong showing among a newer generation, and normal voter fatigue with any party in power, Labour may eventually find its way back to Downing Street, more likely with a minority government. But as now, the party will have difficulty finding willing coalition partners with enough seats of their own to push it over the top.

Divisions over Brexit — the 2016 referendum vote was 52 percent to 48 percent — were only enhanced by this election. The Conservatives, promising a hard Brexit, with Britain out of the European single market and customs union, garnered votes and some seats in areas like the north and West Midlands, that voted heavily to quit the European Union and gave the U.K. Independence Party large votes in 2015. But that tough stance also put off some who had voted to remain.