Prime Minister Theresa May had just suffered three humiliating defeats in Parliament over plans to leave the European Union and, with a critical vote less than a week away, Britain seemed to be facing its biggest political crisis in decades.
So the stage was set for the opposition leader, Jeremy Corbyn, to deliver a knockout blow.
But when he rose to address a rowdy Parliament chamber on Wednesday, Mr. Corbyn, the left-wing leader of the Labour Party, ignored the chaos over Europe, instead using his allotted questions to focus on poverty and welfare changes.
Throughout the interminable debates over Britain’s withdrawal from the European Union, or Brexit, Mr. Corbyn has often attacked Mrs. May, but he has been conspicuously noncommital about what he would do instead.
To this point, Mr. Corbyn’s strategy has served him well.
Yet with Parliament expected to reject Mrs. May’s Brexit plan on Tuesday, the space for dodging uncomfortable issues is rapidly dwindling, and a growing question is where Mr. Corbyn will come down. Whatever his decision, it could have a fateful impact on the result.
“I think Corbyn is the only one who has, at least on paper, all of his options open,” said Steve Fielding, professor of political history at the University of Nottingham. But as early as next week, assuming Parliament votes on Mrs. May’s plan, he will probably be forced to articulate a definitive position on Brexit.
It is generally agreed that Mr. Corbyn’s ultimate goal of forcing a general election is beyond reach. Failing that, he could promote an alternative version of Brexit that keeps Britain more closely tied to the European Union. Or he could push for a second referendum that might reverse Brexit altogether.
To delay further runs the risk of having the decision made for him. “If it comes to it, he will support it,” Mr. Fielding said of the second referendum, “because, by that point, he is out of options and to say ‘no’ is almost impossible.”
But these are not easy decisions for the Labour leader: The party is split on Brexit, albeit not to the degree of the Conservatives. And Mr. Corbyn himself is a lifelong critic of the European Union, so he is reluctant to lead the charge to defeat it.
Most of Labour is more pro-European than the party’s leader. In the 2016 referendum, a majority of Labour voters wanted to remain in the European Union, as do the bulk of party members upon whose support Mr. Corbyn depends.
Yet, many traditional Labour supporters in left-behind areas voted to leave the European Union, attracted by the anti-immigration rhetoric of the pro-Brexit campaigners. Labour lawmakers from these areas want to press on with Brexit, or at least end free movement of workers.
That Mr. Corbyn, 69, has emerged in such a pivotal position is the consequence of one of the great surprises of British politics. For decades, Mr. Corbyn, whose political profile compares to that of Senator Bernie Sanders of Vermont, toiled at the fringes.
He became party leader in 2015 with the support of an expanded membership, but his left-wing agenda alienated many Labour lawmakers. He was vindicated last year when Mrs. May called a general election in hope of fattening her majority in Parliament. She proved a dismal campaigner, however, while Mr. Corbyn surprised many by inspiring a swath of young voters and others who surged to the polls and left the prime minister without a parliamentary majority.
His trick — then as now — was to talk a lot about austerity and very little about Brexit, garnering votes from both sides of the polarized debate on Europe. And Britons generally warmed to a mild-mannered vegetarian who makes his own jam and did not seem as menacing as he had been portrayed by the tabloids.
But now Mr. Corbyn faces momentous decisions on Brexit, ones he probably hoped he could dodge.
“What he really wants is a Brexit which he is not responsible for, so that we are out of the E.U. but he can’t be blamed for it,” Mr. Fielding said. “He wants Brexit without responsibility, though of course he can’t say so. So what he says is that he wants a general election. In that way he says he wants something he can’t get.”
Mr. Corbyn’s views of the European Union are shaped by his years on the far left, which criticized the bloc as a bankers’ club.
He supported Remain in the 2016 referendum but with little enthusiasm, and seemed unperturbed by the outcome. Allies acknowledge that he sees winning power and transforming Britain economically as much more important than resolving the country’s tortured relationship with its continental neighbors.
Leaving the bloc has some advantages, from Mr. Corbyn’s perspective. If a Conservative-led Brexit were to stunt economic growth, that might help Labour win the next election. In that case, a future Corbyn government could cast off current European Union rules that prevent subsidies to loss-making businesses and other restraints.
So, for now, Mr. Corbyn sticks doggedly to the party’s stated policy: it rejects Mrs. May’s deal, and if she is defeated in a vote on Tuesday, Labour will press for a general election. If that fails then all options are open, including a second referendum.
Even in meetings with the senior colleagues of his shadow cabinet, Mr. Corbyn has said little about those options. Labour’s stopgap alternative to Mrs. May’s policy is a “jobs first Brexit”: full access to Europe’s single market without the free movement of workers that comes with it.
Adrian Pabst, a lecturer in politics at the University of Kent, describes this as “Corbyn’s equivalent of Boris Johnson’s ‘having your cake and eating it,’ ” a reference to the unrealistic outlook of Mr. Johnson, the bombastic pro-Brexit Conservative and former foreign secretary.
So things will have to shift. Labour’s top priority is to shut off the option of an economically damaging and disorderly exit without a deal, said Seema Malhotra, a Labour lawmaker and member of the Brexit parliamentary committee.
But Ms. Malhotra also expects pressure to suspend Article 50, the treaty clause under which Brexit takes place in March, and describes a second referendum as a growing possibility. “Ultimately, we don’t know where this is going to go,” she said.
Even some pro-Europeans worry about a second referendum, as they fear giving the public the option of voting for an economically destructive no-deal Brexit. Yet, without that, it will be hard to get a majority of lawmakers to agree to hold a plebiscite.
“There is no consensus among the different factions of the Labour Party on the referendum question,” Mr. Pabst said. “Nor is there a majority in the House of Commons for a People’s Vote — for now,” referring to a second referendum.
Mr. Corbyn’s support could change that. But he would almost certainly pay a political price.
“He is not going to take the initiative until he has to,” Mr. Fielding said. “He doesn’t want to alienate Labour voters, or people who might vote Labour, who voted Leave.”
But after being seen to try everything else, Mr. Corbyn will ultimately take the plunge, Mr. Fielding predicts.
“At least at that point he could say, ‘We wanted a good Brexit, but this is all that’s left now — and now you have to decide.’ ”