By William James, Elizabeth Piper and Kylie MacLellan
LONDON (Reuters) - Prime Minister Boris Johnson puts his last-minute Brexit deal to a vote in an extraordinary sitting of the British parliament on Saturday, a day of reckoning that could decide the course of the United Kingdom's departure from the European Union.
More than three years since the United Kingdom voted 52-48 to be the first country to leave the European project, Johnson will try to win parliament's approval for the divorce treaty he struck in Brussels on Thursday.
In a day of Brexit high drama, lawmakers convene for the first Saturday sitting since the 1982 Argentine invasion of the Falklands, while hundreds of thousands of people are due to march to parliament demanding another referendum.
Johnson depicted the vote as the last chance to secure an orderly Brexit. Though he is obliged by law to seek a Brexit delay if his deal falls, Johnson said the United Kingdom would still leave on Oct. 31. He didn't explain how.
"There have been any number of false dawns. Deadlines for our departure have come and gone," Johnson, the face of the 2016 campaign to leave the EU, wrote in Britain's best-selling newspaper, The Sun. "Today can be the day we get Brexit done."
The so-called Brexit "Super Saturday" tops a frenetic week which saw Johnson confound his opponents by clinching a new Brexit deal although his Northern Irish allies opposed it.
In a divided parliament where he has no majority and opponents are plotting maximum political damage ahead of an imminent election, Johnson must now win the support of 320 lawmakers to pass his deal.
If he wins the vote, Johnson will go down in history as the leader who delivered a Brexit - for good or bad - that pulls the United Kingdom far out of the EU's orbit.
Should he fail, Johnson will face the humiliation of Brexit unraveling after repeatedly promising that he would get it done - "do or die" - by Oct. 31.
Johnson will make a statement to lawmakers on Saturday morning, after which there will be a debate and then votes on amendments and finally - if all goes according to the government's plan - his deal.
'GET BREXIT DONE'
Johnson won the top job by staking his career on getting Brexit done by the latest deadline of Oct. 31 after his predecessor, Theresa May, was forced to delay the departure date. Parliament rejected her deal three times, by margins of between 58 and 230 votes earlier this year.
He said Saturday's vote was the last chance to get Brexit done with lawmakers facing the option of either approving the deal or propelling the United Kingdom to a disorderly no-deal exit that could divide the West, hurt global growth and trigger violence in Northern Ireland.
"Today we MPs (Members of Parliament) have the chance to free you from the never-ending Brexit saga and move this country forward," Johnson wrote. "In less than two weeks, on October 31, we would be out of the EU."
To win the vote, though, Johnson must persuade enough Brexit-supporting rebels in both his own Conservative Party and the opposition Labour Party to back his deal. His Northern Irish allies and the three main opposition parties oppose it.
In a boost for Johnson, some influential hardline Brexit supporters such as Mark Francois and Iain Duncan Smith said they would support the deal.
Steve Baker, the head of a hardline Brexit faction in the Conservative Party, has told his European Research Group allies that they should vote for Johnson's deal.
Yet Johnson's opponents have put forward amendments that could wreck his path to Brexit.
BREXIT UP IN THE AIR
One expelled Conservative lawmaker, Oliver Letwin, has proposed that the decision on whether to back a deal be deferred until all the legislation needed to implement it has been passed through parliament.
This proposal, which has cross-party support, will be put to a vote at the end of Saturday's debate if selected by Speaker John Bercow. If the amendment is approved by parliament, Johnson's deal would not then be put to a vote on Saturday.
That would be a major setback that would leave Brexit, yet again, up in the air as hundreds of thousands of protesters rally outside parliament to demand another referendum.
The Letwin move, aimed at preventing the United Kingdom from somehow dropping out of the EU without a deal, would in effect force Johnson to delay Brexit until all the laws needed to leave have passed through parliament's multi-stage approval process.
Even though Johnson believes this can be achieved by Oct. 31, others think it would need a short 'technical' delay.
A law passed by Johnson's opponents obliges him to ask the EU for a Brexit delay until Jan. 31 2020 unless he has secured approval for his deal by the end of Saturday.
"My aim is to ensure that Boris’s deal succeeds," Letwin said. "But that we have an insurance policy which prevents the UK from crashing out on 31 October by mistake if something goes wrong during the passage of the implementing legislation."
If Johnson's deal is rejected, there may also be a vote on whether to leave without a deal and also whether to hold another referendum.