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.
Seeking to explain the perceived reversal of Mrs. May’s fortunes, Anthony Wells, research director at YouGov polling, said Mrs. May had erred by failing to present a proactive narrative, much in the same manner as Hillary Clinton was seen by some as defining herself as a foil to Donald J. Trump without adequately explaining what she stood for.
“The Conservatives don’t seem to have a strong message to their campaign,” he said. “May has fashioned herself as ‘not Corbyn’ without explaining why people should vote for her. At the same time, she has made some very damaging U-turns, while Corbyn has not proven to be the scary monster that some had feared.”
Mrs. May, who has sought to portray herself as a compassionate conservative, provoked the ire of many voters when, during this month’s publication of the Conservative manifesto, she was forced to retreat from an unpopular proposal to make older Britons shoulder more of the costs of long-term home care. The proposal, derided as a “dementia tax,” undermined her image as the champion of those “just about managing” to get by financially.
Adding ammunition to rivals seeking to portray her as a Scrooge, the conservatives proposed a less advantageous system for automatic raises in pensions and contentious plans to scrap universal free school lunches for children.
In contrast, the Labour Party’s manifesto offered many populist measures, among them increasing funding for the National Health Service and a pledge to scrap tuition fees for students starting college in September.
Peter Kellner, a leading political analyst and polling expert, said the narrowing of the polls at least partly reflected that Mr. Corbyn appeared to be resonating with young people and women, who welcomed his calls for more money for the country’s strapped social services.
He said many initially reluctant Labour voters, who had voted for the party in the past but had been wavering, had also decided to stay. “A lot of people regarded Corbyn as useless and extreme and not up to running the country,” he said. “But when exposed to him over the past few weeks, a number have found him not so bad after all.”
Mrs. May’s conspicuous absence at Wednesday’s debate overshadowed the event as her opponents criticized her for running scared. “How dare you call a general election and run away from the debate,” Tim Farron, leader of the Liberal Democrats, said, chiding her openly. Addressing voters, he added: “You’re not worth Theresa May’s time. Don’t give her yours.”
The Scottish National Party deputy leader, Angus Robertson, also criticized her for not having the “guts” to face voters.
Mrs. May’s absence threatened to solidify a growing perception of her, fairly or not, as remote and arrogant. On Monday, she had already declined to appear on the same stage to debate Mr. Corbyn, resulting in an awkward spectacle in which both candidates were questioned separately.
That had followed a series of embarrassing flip-flops — not the least her call for early elections after insisting she would not — that were undercutting her carefully cultivated image for straight-talking honesty. A satirical song about her, “Liar, Liar,” shot to the top of the charts in Britain this week, an indication that the actions were taking a toll.
Referring to Mrs. May’s decision not to attend the debate — and invoking former Prime Minister Margaret Thatcher’s famous phrase of steadfastness, “the lady’s not for turning” — Angela Rayner, who represents Labour on education policy, said, “This prime minister is for turning, but not for turning up,” according to The Financial Times.
Whoever wins, Ed Miliband, the former Labour leader who lost badly in 2015 after predictions of a close race, warned against giving credence to the pollsters, who had shown their fallibility.
“The pollsters have been off my Christmas card list since 2015,” he wrote on Twitter.