LONDON — In the pre-dawn hours of Friday, Prime Minister Boris Johnson learned that his Conservative Party had crashed to defeat in a district it had represented for more than a century. Twelve hours later, Britain reported more than 90,000 new cases of Covid-19 as the Omicron variant engulfed the country.
Each of those events would be daunting enough on its own. Together, they pose a uniquely difficult challenge to Mr. Johnson as he struggles to navigate his nation through the latest treacherous phase of the pandemic.
The electoral defeat exposed the vulnerability of a prime minister who built his career on his vote-getting skills. Normally reliable Conservative voters turned on the party in striking numbers, disgusted by a steady drip of unsavory ethics disclosures and a growing sense that the government is lurching from crisis to crisis.
The defeat came on top of a mutiny in the ranks of Conservative lawmakers, around 100 of whom voted against Mr. Johnson’s plan to introduce a form of Covid pass in England earlier in the week. Having been politically rebuked, he now has less flexibility to impose new restrictions to curb a virus that is spreading explosively.
Mr. Johnson is betting he can avert a full-blown crisis by massively accelerating Britain’s vaccine booster program. But so far, the rate of infections is outrunning the percentage of people getting their third shots. With the variant doubling every 2.5 days, epidemiologists warn that some type of lockdown might ultimately be the only way to prevent an untenable strain on hospitals.
“What on earth is the prime minister going to do if the rising Covid numbers means he is getting strong scientific advice to take further restrictive measures?” said Jill Rutter, a senior research fellow at UK in a Changing Europe, a research institute.
Mr. Johnson was able to pass his recent measures thanks to votes from the opposition Labour Party. But that dramatized his political weakness, Ms. Rutter noted, and resorting to it again would further antagonize his own rank and file. “That’s politically a terrible place for the prime minister to be,” she added.
Indeed, Mr. Johnson needs to worry about fending off a leadership challenge — a once-remote scenario now suddenly plausible as Conservative lawmakers worry that the calamitous result in North Shropshire, a district near England’s border with Wales, could translate into defeat in the next general election.
The victorious Liberal Democrat candidate, Helen Morgan, overturned a majority of almost 23,000 won by the former Conservative lawmaker, Owen Paterson, at the last general election, in 2019. Mr. Paterson, a former cabinet minister who had held the seat since 1997, resigned last month after breaking lobbying rules, despite an unsuccessful effort by Mr. Johnson to save him.
About the only reprieve for Mr. Johnson is that Parliament recessed for the Christmas holiday on Thursday. That will temper the momentum behind any possible leadership challenge, at least until Conservative lawmakers return to Westminster after the New Year and assess the state of their party and the country.
A prime minister who just a week ago was promising to save Christmas may now need Christmas to save him.
“I totally understand people’s frustrations,” Mr. Johnson said on Friday. “In all humility, I’ve got to accept that verdict.” But he also blamed the news media, telling Sky News, “some things have been going very well, but what the people have been hearing is just a constant litany of stuff about politics and politicians.”
Mr. Johnson’s standing has been weakened by claims, widely reported in the papers, that his staff held Christmas parties in Downing Street last year at a time when they were forbidden under coronavirus restrictions.
The cabinet secretary, Simon Case, had been investigating those allegations but on Friday evening, he abruptly withdrew after a report surfaced that he was aware of a separate party held in his own office last year. Though another civil servant, Sue Gray, will take over the investigation, the latest disclosure is only likely deepen to public suspicion about the government’s behavior.
Even before the election loss in North Shropshire, there was speculation that Mr. Johnson could face a formal challenge to his leadership, little more than two years after he won a landslide election victory in December 2019.
To initiate a no-confidence vote, 54 lawmakers would have to write to the chairman of the committee that represents Conservative backbenchers, Graham Brady. Such letters are confidential, but analysts do not believe that prospect is close.
Even so, Friday’s result will increase jitters in Downing Street. North Shropshire was one of the Conservative Party’s safest seats, in a part of Britain that supported Brexit, Mr. Johnson’s defining political project. Many Labour Party voters and others hostile to the Conservatives coalesced around the Liberal Democrats, the party deemed most likely to defeat the Tories in that region — a practice known as tactical voting.
Were this to be repeated nationally in the next general election it could deprive the Conservatives of perhaps 30 seats and, in close contest, affect the outcome, said Peter Kellner, a former president of the polling firm YouGov.
“Tactical voting has a chance to make a material difference to the politics of Britain after the next general election,” he said.
In recent weeks, Labour has moved ahead of the Conservatives in several opinion surveys, which also recorded a steep drop in Mr. Johnson’s approval ratings. Political analysts said that could also put the prime minister in a vulnerable position, given the transactional nature of his party.
“The Tory Party is a ruthless machine for winning elections,” said Jonathan Powell, a former chief of staff to Prime Minister Tony Blair. “If that is continuing into an election cycle, the party will get rid of him quickly.”
But while the political climate remains volatile, most voters are more preoccupied by the effect of the Omicron variant as they prepare for the holiday season. Scientists said it was too soon to say whether the variant was less severe than previous ones, but they warned that even if it was, that would not necessarily prevent a swift rise in hospital admissions, given the enormous number of infections.
“If you have enough cases per day, the number of hospitalizations could pose potentially great challenges for any hospital system,” said Neil Ferguson, an epidemiologist at Imperial College London, whose frightening projections about the virus prompted Mr. Johnson to impose his first lockdown in March 2020.
Ms. Rutter said Mr. Johnson could yet emerge unscathed if the variant is milder than feared, hospitals are not overwhelmed, and the booster program is effective. Earlier this year, his fortunes revived when Britain’s vaccination rollout was fast and effective, allowing him to remove all restrictions in July.
By weakening Mr. Johnson’s position, however, the defeat in North Shropshire is also likely to embolden his rivals, among them the chancellor of the Exchequer, Rishi Sunak, and the foreign secretary, Liz Truss. Any resulting tensions within the cabinet are likely to erode Mr. Johnson’s authority further.
All of that is a dangerous recipe for a prime minister who may find himself forced to return to Parliament to approve further restrictions.
“In March 2020, he had massive political capital coming off the back of that fantastic election victory,” Ms. Rutter said. “He’s managed in that time to pretty much squander that political capital, certainly within his party.”