A Brexit deal in hand, Boris Johnson faces an uphill struggle in parliament

But nobody really knows how things will pan out. “We have one step forward, in that we’re talking about something substantive,” said Sam Lowe, a senior research fellow at the Centre for European Reform, a research group in London. “But we still really have no idea where this is all going to land.”

Mr. Lowe said that Mr. Johnson faced a difficult, but not impossible, task in getting his plan through, and that he could even gain by losing. “I think he could lose, in which case this will all be about positioning himself for a general election,” Mr. Lowe said.