On Wednesday, Mr. Corbyn shook up the race with a last-minute decision to join a televised debate in Cambridge that Mrs. May did not attend. Emboldened after a competent performance in a television event on Monday, Mr. Corbyn sought, before the debate, to portray Mrs. May as weak and evasive for not appearing.

“Refusing to join me in Cambridge tonight would be another sign of Theresa May’s weakness, not strength,” Mr. Corbyn, who has been trying to revamp his own image as a weak-kneed pacifist, said in a statement.

In explaining why she would not be participating, Mrs. May said she had been facing Mr. Corbyn week after week in the ritual prime minister’s questions in Parliament, and had also been taking questions from voters directly on the campaign trail.

The home secretary, Amber Rudd, represented the Conservatives in the televised debate.

Just six weeks ago, Mrs. May had a lead of as much as 24 percentage points over Mr. Corbyn in some polls. Pundits were already asking if Mr. Corbyn, a gaffe-prone leftist viewed by many in his own party as unelectable, would step down if he lost the election. “Theresa on the March,” proclaimed the headline in The Sun, a popular tabloid.

Now, though, after initially casting herself successfully as the only “strong” and “stable” leader qualified to lead Britain as it exits the European Union, Mrs. May appears to have alienated many voters through a mix of hubris and austerity policies. At the same time, Mr. Corbyn, the beneficiary of subterranean expectations, appears to have been given a lift by simply not messing up badly.