Mr. Johnson has sought to use the threat of a no-deal Brexit as leverage in talks with the European Union. His office railed against the opposition’s plan, calling it “utterly perverse.”

But the prospect of a new Brexit deal before Oct. 31 remains slim. And for advocates of staying in the European Union, the word that lawmakers opposed to a no-deal exit were ready to collaborate was a welcome sign.

Reality: The strategy will face considerable hurdles when Parliament reconvenes in September.

Next: If the plan agreed to on Tuesday succeeds in tying Mr. Johnson’s hands, it could give him all the reason he needs to try calling an early general election — giving him a chance to expand the Conservatives’ thin working majority. But he needs Labour’s backing to do so.

Quotable: “We’re still in that situation where there are all sorts of exits out of the burning building, but they all look locked,” said one politics professor.