LONDON — Within minutes after Prime Minister Theresa May’s plan for leaving the European Union was resoundingly defeated on Tuesday night, the leader of the opposition Labour Party, Jeremy Corbyn, rose in Parliament and vowed in thunderous tones to unseat her with a motion of no confidence.
For Mr. Corbyn, a soft-spoken outsider who stormed to the head of the opposition party three years ago, the moment marked a crossroads. After years of lying low on the question of Brexit, steadfastly refusing to commit to a clear course, he finally made his move, thrusting himself into the center of the debate, where he will have to choose a side.
Having risen to the edge of power by presenting himself as an authentic, left-wing champion of grass-roots Labour members, it is increasingly untenable for Mr. Corbyn to defy them and avoid a decision on Brexit.
“The danger here is that the shine is coming off the Corbyn project because of his triangulation on Brexit,” said Michael Chessum of Another Europe Is Possible, a left-wing group campaigning for Labour leaders to back a second referendum on Brexit. “If people start to think that Corbyn is just another politician like all the other politicians, that is the thing that will kill the Corbyn project.”
The timing for a decisive move, after Mrs. May’s government had just suffered the worst parliamentary defeat in Britain since the 19th century, seemingly could not be more propitious.
Mr. Corbyn’s no-confidence motion will almost certainly fail on Wednesday, returning Parliament to a state of paralysis. But he will be in a position to exercise enormous influence over whatever plan emerges from the next tumultuous weeks — if he defines his stance.
If he decided to back a “soft” Brexit, maintaining close ties with the European Union, that proposal would probably pass. If he backed a second referendum that could thwart Brexit altogether, that would have at least a chance of passing. If he stood aside and let Labour lawmakers back a version of Mrs. May’s deal, that, too, would be likely to pass.
And if he turned his back on all those options, he could drastically raise the chances of Britain leaving without a deal, risking a recession and perhaps even shortages of food and medicine.
Still, the choice is not an easy one. The debate over Brexit within the Labour Party, while milder than the warfare among Mrs. May’s Conservatives, is dangerously divisive. To achieve his ultimate goal of returning Labour to power and reversing decades of neoliberal policies, Mr. Corbyn needs a united party.
That presents him with a dilemma. As a lifelong critic of the European Union, which he has portrayed as a bankers’ club that blocks left-wing policies, Mr. Corbyn is loath to reverse Brexit and anger working-class Labour voters who opted to leave. But the Labour activists who powered his unlikely leadership bid are putting enormous pressure on him to do just that.
The predicament, growing more urgent by the day, mirrors questions confronting left-wing parties in Europe and the United States about how to fight populist movements that trade on anti-immigrant sentiment.
Should they stand up for open borders and multiculturalism and risk cutting loose white workers who have drifted to the anti-immigrant right? Or try winning back those voters with a liberal version of the crusade against global institutions, trade pacts and migration?
“If you look around Europe, center-left parties are facing a dilemma,” said Tim Bale, a professor of politics at Queen Mary, University of London, who has surveyed Labour membership on Brexit. “How do they maintain or even get back an electoral coalition of workers and middle-class, more educated voters?”
The daunting electoral math facing Labour has given Mr. Corbyn reasons for remaining vague.
At the most recent general election, in 2017, the party achieved unexpected victories in prosperous, pro-European parts of England. But if it is to form a government after the next election — scheduled for 2022, should Mrs. May’s government survive until then — it would also need the support of pro-Brexit voters in small towns who remained loyal to the Conservatives last time.
Mr. Corbyn also has allies within Labour cheering his strategy of not taking any particular side. Like him, they rank membership in the European Union low among their priorities, below the goals of ending austerity and pulling Britons out of poverty.
“Brexit is actually a relatively small issue compared to the social issues facing this country,” said Callum Cant, a Labour member and Corbyn backer. “Regardless of people’s position on the E.U., we need to fight together for a larger project of social transformation.”
A recent poll shows Labour would have the support of only a fifth of voters if it backs Brexit, and roughly a third of voters if the party instead campaigns for a second referendum.
But even in seats that voted to leave the European Union, it is essential that Labour keep the backing of pro-Europeans, who are more likely to be in the party’s camp, said Robert Ford, a professor of politics at the University of Manchester. And winning swing voters does not necessarily mean caving to anti-immigrant sentiment, he said.
Just as American voters have moved to the left on immigration under President Trump, he said, Britons’ feelings have softened since the Brexit referendum.
“In a strange sort of way, the anti-immigration side winning at least temporarily seems to have the paradoxical effect of shifting opinion the other way,” he said.
While Labour members have largely stuck behind Mr. Corbyn so far, roughly two-thirds voted to remain in the European Union in the 2016 referendum, and nearly three-quarters would like a rerun, according to a study led by Professor Bale.
Thousands of them have submitted notes recently to a group that includes top lawmakers, some of them threatening to leave the party over Brexit and describing “despair” over Mr. Corbyn’s months of equivocating. Pro-European party activists have circulated a resolution saying Labour should demand a second referendum and campaign to stay in the bloc; up to 200 local parties are expected to debate it by the end of the month.
Could he defy them and vote through a Brexit plan? Grace Blakeley, another Labour member and a representative on the party’s National Policy Forum, said Mr. Corbyn would never explicitly support a government Brexit proposal, but might watch as Labour lawmakers defected to get behind such a deal.
She described the Labour leader’s strategy as angling for a populist movement that could win over pro-Brexit voters not by accommodating anti-immigrant views, but rather by making an economic argument for overturning the status quo.
“The most successful campaigns of recent years,” she said, “have been left-populist campaigns that take the existing economic anger and rather than mobilizing it against migrants, mobilize it against elites and the establishment that have rigged the economy.”