The UK’s 650 members of Parliament will convene in Westminster for only the fourth Saturday sitting since World War II to decide whether to endorse the Brexit agreement Johnson struck Thursday with the European Union.
If the British prime minister wins the critical vote, the country will be on course to leave the EU at the end of the month in an orderly break-up.
A status-quo transition period lasting until the end of 2020 will give businesses and citizens time to plan for life outside the EU.
But if Johnson loses, the UK will be hurled into an unprecedented political and constitutional crisis, with a potentially severe impact on trade, hitting supplies of food and fuel, and sparking civil disorder.
Johnson has vowed he will seek to force the UK out of the EU on Oct 31 even if MPs reject his divorce accord Saturday — without an agreement to soften the impact on the economy and society.
His opponents in Parliament are aiming to stop him by using a new law under which he is required to write to the EU by the end of the day Saturday seeking to secure an extension to the Brexit deadline, if he hasn’t won MPs’ backing for his deal, or for a no-deal split.
Johnson says he will keep fighting to exit the EU on time, whatever the cost. The clash could end up with legal challenges against the government within days, potentially concluding in the UK Supreme Court.
An emergency 11th hour summit in Brussels, a general election, another attempt to pass a vote in Parliament, and even another referendum could all follow.
Without a majority in the Commons, and having alienated his Northern Irish allies, who prop up his government, Johnson faces a struggle to secure the votes he needs to prevail.
He spent Friday trying to woo sceptical MPs in his own party as well as opposition Labour politicians who represent pro-Brexit districts.
The result looks set to be too close to call, and Johnson’s team believe they’re making progress, according to an official who asked not to be named.
“Imagine what it could be like tomorrow evening, if we have settled this, and we have respected the will of the people, because we will then have a chance to move on,” Johnson said in an interview with BBC TV on Friday.
“This has been a long, exhausting and quite divisive business.”
Johnson received a boost from the EU when French President Emmanuel Macron said even if Parliament rejected the deal, there would be no further delay to Brexit day beyond the current deadline of Oct 31.
That added to the pressure on MPs who would prefer another extension to the deadline to leaving the EU without a deal.