“Honestly, I am completely torn. I struggle with it all the time,” said Rory Stewart, one of 21 members of Parliament expelled from the Conservative Party last month for defying Mr. Johnson by trying to prevent Britain from leaving the European Union without an agreement.

“The temptation would be to say I reject it because Theresa May’s deal is better,” Mr. Stewart said. “But at the same time, realistically, Theresa May’s deal is not coming back. So if you reject this, what are you really laying yourself open for? No deal, second referendum or endless delay.”

To add to the sense of confusion, Parliament might be asked on Saturday to consider an amendment put forward by Oliver Letwin, a lawmaker who was expelled from the Conservatives for defying Mr. Johnson over Brexit. It would withhold approval of the Brexit deal until Parliament passed legislation to enact it. That condition would force Mr. Johnson to ask the European Union for a further postponement of the Brexit deadline, to Jan. 31.

Most lawmakers are entrenched on one side or the other of the Brexit debate, so the outcome may be decided by three critical blocs: the “Spartans” and other hard-line, pro-Brexit Tories, and rebels from both major parties, the Conservatives and Labour.

The Spartans

These are the 28 hard-line pro-Brexit Conservative lawmakers who voted against Mrs. May’s last attempt to get her deal through Parliament (and are a subset of the so-called European Research Group, which has around 80 members). If the Spartans can be persuaded to back Mr. Johnson’s deal, they will get him close to the line since the rest of the larger group is expected to largely support the plan.