What is the showdown about?

Britain is scheduled to leave the European Union on Oct. 31, and Mr. Johnson has vowed that it will happen on time, with or without a deal. But opposition parties and a significant number of rebels in his Conservative Party are adamantly opposed to a no-deal Brexit, which they say would be chaotic and economically damaging in the short and long term.

Seeking to tie Parliament’s hands, Mr. Johnson said last week that he had asked the queen to prorogue, or suspend, Parliament later this month, cutting short the already tight time frame for lawmakers to come up with a way to prevent Britain from crashing out of the European Union with no agreement in place.

The move left lawmakers scrambling for a way to weigh in on the most momentous decision the country has faced in its recent history. They decided to ram through legislation requiring the government to seek a Brexit extension if it has not reached an agreement with the European Union on withdrawal by the deadline.

Mr. Johnson has made it clear that he was unalterably opposed to seeking another extension, and on Tuesday he reiterated his plan to call for snap elections if the legislation were approved.

“I don’t want an election, the public don’t want an election, but if the House votes for this bill tomorrow, the public will have to choose who goes to Brussels on Oct. 17 to sort this out and take this country forward,” Mr. Johnson said, referring to the next European Union summit, which is seen as the final moment for a deal to be reached.