In the final stretch of the campaign, the Labour Party has shifted to a more defensive posture. That has meant pouring precious resources into traditional strongholds in the north and middle of the country — places that voted to leave the European Union in the 2016 referendum — rather than trying to win Conservative districts in the south that voted to remain.

The hope is that Labour can lure undecided Britons and squeeze the vote of the centrist Liberal Democrats, whose campaign seems to have faltered. As Election Day approached, many opinion polls tightened, jangling nerves among Mr. Johnson’s team.

Still, many Labour supporters had expected more, considering that nine years of Conservative rule have produced tough and unpopular austerity policies and more than three years of political chaos over Brexit.

“There is now some panic about whether Labour can hold some of their existing seats,” said Steven Fielding, a professor of political history at the University of Nottingham. “It has now become a defensive campaign, and from the perspective of what we had been led to believe, that is a clear defeat.”

Mr. Corbyn’s leadership has been marred by factionalism and by persistent claims that he has tolerated anti-Semitism within Labour’s ranks. The critics include the chief rabbi, Ephraim Mirvis, the spiritual leader of much of Britain’s Orthodox Jewish community, who argued that “a new poison — sanctioned from the very top — has taken root.”