Depending on what happens this week, historians will begin to situate Mrs. May on the continuum of British leaders, somewhere between Anthony Eden (worst) and Winston Churchill (best). In my latest article, I submit the following: Mrs. May made a genuine effort to lead her fractured country to a compromise, but she did it too late. By devoting her early months in office to reassuring the hard-Brexit faction of her party that she was one of them, she sacrificed a chance to build trust with moderates.

“It’s not enough to turn around after two and a half years and say, ‘I’m a unifier,’” said Ian Dunt, the author of “Brexit: What the Hell Happens Now?”

The situation provides the perfect opening for the leader of the opposition, Jeremy Corbyn, to step in with moral guidance, right? Not exactly.

My colleague Stephen Castle describes Mr. Corbyn’s pathological ducking and weaving on the most momentous issue facing the country. The reason is obvious enough: Mr. Corbyn, shaped by his years on the far left, always liked the idea of leaving the E.U., but a majority of Labour voters want to remain.

After Parliament votes on Mrs. May’s withdrawal agreement, Mr. Corbyn’s time for prevarication will run out. Steve Fielding, a political historian, bets he would back a second referendum.

“If it comes to it, he will support it,” he said. “Because, by that point, he is out of options and to say ‘no’ is almost impossible.” And so, readers, onward to Tuesday’s vote. Pass me the peanut butter, and a spoon.