• With Britain in political crisis and a new deadline to leave the European Union two weeks away, Parliament is debating Prime Minister Theresa May’s Brexit plan, which it has already rejected twice by large margins, before a third vote on Friday afternoon.
• Lawmakers are to vote around 2:30 p.m. London time on the 585-page agreement, which details Britain’s relationship to the European Union through the end of 2020, while longer-term plans are sorted out.
• If it does not pass, as seems likely, Britain’s withdrawal is set to take place on April 12 without an agreement — the “no-deal” Brexit that many economists and officials have warned would do serious economic damage.
• In a bid to win over hard-line Brexit supporters, Mrs. May promised Conservative lawmakers this week that she would step down as prime minister if the deal were approved. The prime minister is hoping that enough lawmakers will reverse course, despite their concerns, rather than risk crashing out without a deal.
What exactly are lawmakers voting on?
Parliament has twice rejected Mrs. May’s proposal, but this time there is a twist: Lawmakers will only vote on the withdrawal agreement, the legally binding part of the deal.
They will set aside, for now, a decision on the nonbinding “political declaration,” a statement of what both sides want in Britain’s long-term relationship with the European Union. The two parts were separated to get around a procedural rule that would have prevented Mrs. May from making a third attempt to get the deal through.
The withdrawal agreement sets the terms of a transition period after Britain leaves the bloc, while long-term arrangements are negotiated. It would last through the end of 2020, but could be extended for two years.
It lays out in detail the nation’s trade relationship with the bloc, keeping Britain tied, at least temporarily, to many European Union tariff and product rules. It allows Britain to set its own immigration rules, but protects the bloc’s citizens who are already living in Britain.
This agreement also includes language dealing with the border between Ireland, a European Union member country, and Northern Ireland, a part of the United Kingdom — a confounding and divisive issue that has proved to be the biggest sticking point in Parliament.
At the moment, goods and people flow freely between Ireland and Northern Ireland. Under the withdrawal agreement, that arrangement would continue even if the two sides have not reached a long-term pact by the end of 2020, under a provision known as the backstop.
The backstop would keep Britain, and particularly Northern Ireland, tied to many European Union rules, to avoid building physical barriers on the border. That is anathema to many Brexit supporters, who fear that it could leave Britain permanently beholden to the bloc.
Long odds and enormous stakes for Theresa May
Opposition to Mrs. May’s deal has been decreasing among members of her warring Conservative Party, and she was expected to make a last-ditch plea to lawmakers before the vote on Friday.
But her odds are not good, and the stakes for her are enormous.
Brexit has been the defining issue of her tenure, but the agreement that Mrs. May negotiated with the European Union was overwhelmingly rejected in two previous votes in Parliament.
She surprised lawmakers on Wednesday by telling them that she would step down if her plan passed this time, while warning that another rejection could lead to a long postponement that might kill Brexit entirely.
As she hoped, her offer to step down won over some hard-liners in her party who disparage her and want a more complete break with Europe and fear a long delay.
But so far, there has been no sign that she has swayed the Democratic Unionist Party, a small but influential Northern Ireland group that has so far opposed the agreement.
In the last vote, on March 13, Parliament rejected Mrs. May’s deal, 391 to 242. Nearly all opposition lawmakers, 75 Conservatives and all 10 from the D.U.P., voted against it.
Friday’s vote will no doubt be closer, but Mrs. May needs to change the minds of about 75 lawmakers to win — a very tall order.
Another rejection would take the process back to square one. Parliament would still be struggling to agree on a way forward. And Mrs. May, though further weakened, would still be prime minister, with increased speculation about a possible general election.
What comes next?
Like many moments in Britain’s prolonged journey, it’s not entirely clear.
Britain was originally set to take officially leave the European Union on Friday, but European leaders agreed last week to a short extension, laying out three possibilities:
If Parliament accepts Mrs. May’s deal, Britain would leave the bloc on May 22.
If lawmakers reject it again, and Britain takes no further action, it would withdraw on April 12 without an agreement — an option wanted by neither the European Union nor most British lawmakers.
Mrs. May could once again ask Brussels for more time if the plan is rejected. But European leaders have said that they would be open in such a case only to a long extension, possibly of a year or more, to allow for a fundamental rethinking of Britain’s position.
With Mrs. May’s promise to step down, approval of the agreement would set off a fight among Conservatives to choose a new leader.
The deadlock in London could also lead to early parliamentary elections, or a second referendum on Brexit.
Many people in Britain and on the Continent are getting tired of the uncertainty. Among them is Jon Worth, a political consultant who has been making (and remaking) flowcharts to map the potential outcomes of the withdrawal process.
Mr. Worth, who works as a communications consultant for European politicians, has made 22 versions of his Brexit flowcharts, mapping every twist and turn in the political saga.
Who could be the next prime minister?
“I have heard very clearly the mood of the parliamentary party,” Mrs. May told Conservative lawmakers gathered in a meeting room in Parliament this week, as she announced plans to step aside if her Brexit plan were approved. “I know there is a desire for a new approach, and new leadership, in the second phase of the Brexit negotiations, and I won’t stand in the way of that.”
After the surprise offer on Wednesday, political analysts were quick to speculate about who might replace her. Her departure, which would not come before May 22 withdrawal date, would leave the Conservative Party to select a new leader to see the process through.
Candidates for party leadership have to be nominated by two other members of Parliament, though if there is only one candidate, he or she automatically becomes the new leader. If more than two candidates emerge, lawmakers vote among themselves to narrow the field and then put two candidates to a vote by all party members, not just those in Parliament.
There is no obvious front-runner, but British bookmakers are already offering odds on some of the politicians they believe to be probable contenders for the job. They include hard-line Brexit supporters, vocal critics of the prime minister’s approach and supporters of her strategy.