Though the foreign secretary, Dominic Raab, on Sunday predicted that there would be enough support for Mr. Johnson’s plan, the numbers are so close that no one can be certain of the outcome.

Recap: On Monday, in an instance that was familiar to Mr. Johnson’s predecessor, Theresa May, the speaker of the House of Commons, John Bercow, put a stop to a vote on Mr. Johnson’s Brexit plan, saying that Parliament could not be made to vote on the same thing twice (the first time being Saturday).

What’s next: If Mr. Johnson’s plan wins, the government will face a second, critical test when it puts forward an aggressive, accelerated timetable for the legislation, designed to ram it through Parliament in three days.

If Parliament rejects that timetable, it will be almost impossible to avoid another delay. And it would give opponents time to amend the legislation, like adding a referendum on the plan or a customs union stipulation.