What to Watch For in Today’s Brexit Vote: Live Updates

Showdown in Parliament on a new Brexit deal.

Three times before, Britain’s Parliament took up a thorny divorce agreement between Britain and European Union. And three times before, Parliament resoundingly voted it down.

The first deal was presented by Prime Minister Boris Johnson’s predecessor, Theresa May. And the process has driven Britons to anger, frustration, relief and despair. But on Thursday, Mr. Johnson announced that he and European leaders had agreed on a new Brexit deal, creating the potential for a breakthrough in the nation’s yearslong quagmire.

Now, he must get British lawmakers to approve it. Parliament is sitting in a special “super Saturday” session for the first time since the invasion of the Falklands in 1982. The prime minister, who has vowed to get the country out of the bloc by the deadline, Oct. 31, has worked the phones, lobbying, cajoling and pleading with lawmakers to back him.

On Saturday morning, Mr. Johnson urged members of Parliament to back his deal, saying he hoped the historic sitting of the House of Commons “is the moment we can finally” resolve Brexit.

“Now is the time for this great House of Commons to come together,” he added, “as I believe people at home are hoping and expecting.”

He is presenting his deal to lawmakers, after which they will begin the debate. There are likely to be impassioned speeches from Conservative and opposition leaders. Around 2:30 p.m., the first vote is set to occur.

Mr. Johnson needs 320 votes to pass his deal, and the vote is too close to call. The House of Commons has 650 seats. Minus those who do not vote — the speaker, the three deputy speakers and the seven members of Northern Ireland’s Sinn Féin, who do not take their seats in Parliament — that leaves an electorate of 639 lawmakers.

Mr. Johnson’s odds are complicated by the fact that he does not have a working majority in Parliament and has not won a major vote there in the three months he has been in office.

Prime Minister Boris Johnson appears to be betting that enough lawmakers will be fed up with the three years of haggling and will hold their noses and vote for his Brexit deal, seeing it as better than the alternatives of a no-deal exit or no Brexit at all.

But even if he loses, analysts say, Mr. Johnson may be hoping that his ability to bring home a deal will win him a public mandate to pull Britain out of the bloc as swiftly as possible.

Then, he is likely to call for a general election in the coming weeks.

Lawmaker’s last-ditch amendment is accepted.

Some lawmakers who support Prime Minister Boris Johnson’s Brexit deal don’t trust him or his hard-line Brexit backers, fearing that their colleagues could pull a procedural trick to force Britain to crash out of the European Union without a deal.

So a former Conservative lawmaker, Oliver Letwin, whom Mr. Johnson had kicked out of the party, will put forward an amendment as sort of an insurance policy to make approval of the deal conditional on also passing necessary legislation.

In essence, the so-called Letwin Amendment, chosen by the speaker of the House of Commons, John Bercow on Saturday, would turn Parliament’s up-or-down vote on Mr. Johnson’s deal into a much weaker motion, and Saturday would not be the day that lawmakers will fully endorse or reject the Brexit deal.

Mr. Johnson would be forced by law to send a letter to the European Union on Saturday night to request an extension of the Brexit deadline, currently Oct. 31.

Then, before Brexit could happened, lawmakers would get to not only vote on Mr. Johnson’s deal, but also to debate, amend and vote on actual legislation putting that deal into law.

Large march is planned in London to demand a ‘final say.’

Thousands of protesters were gathering in London on Saturday morning to demand a another referendum on Brexit — a show of defiance as British lawmakers prepared to vote on a deal outlining the nation’s exit from the European Union.

Organizers of the People’s Vote March said they hoped to draw more than one million people, which would make it one of the largest demonstrations on record in Britain.

“We are now reaching a crucial moment in the Brexit crisis,” the organizers said in a statement. “The government has adopted the slogan ‘Get Brexit Done’ to try and browbeat an exhausted public into accepting whatever botched Brexit Boris Johnson presents to them but we know this slogan is a lie.”

This is not the first time that throngs of people have taken to the streets over Brexit. A year ago, about 700,000 gathered in advance of a previous deal reached by Theresa May, who was then prime minister.

But even as the protesters were assembling on Saturday, for the supporters of Brexit, the people have already had their say.

It has been more than three years since voters in Britain narrowly supported a referendum to leave the European Union. Those three years have been marked by growing public anger, with the crisis upending friendships and tearing families apart.

Reporting was contributed by Stephen Castle, Mark Landler, Ben Mueller, Marc Santora, Alan Yuhas and Megan Specia.