Facing a critical vote by British lawmakers on her plans for leaving the European Union, or Brexit, the question for Prime Minister Theresa May seems to be not whether she will lose but by how much.

Britain's Parliament is gripped by a mood of anxiety and suppressed excitement at the prospect of a grave political crisis that looms if, as analysts expect, May fails to persuade lawmakers to support her Brexit plan.

If she loses the vote Dec. 11, it will be a significant moment for May and her country, and the margin could be crucial. A crushing defeat could force May to abandon her deal, crash her government or face a bitter power struggle that could force her out.

A more modest loss could keep May in the game, perhaps playing to her well-known strengths of persistence and dogged determination, and allowing her to return to hold a second parliamentary vote.

In that scenario, May might renegotiate small parts of her deal in Brussels while her party whips at home threaten and cajole rebels. Then some opposition lawmakers would need to be persuaded to vote for her plan for fear of a disastrous exit without any deal.

"Although there are no rules about the scale of defeats, if May loses by more than 40 to 50 votes, her deal would be dead," though she could perhaps try to negotiate a different type of exit, wrote Mujtaba Rahman, managing director, Europe, for the Eurasia Group. "If her loss is sizable, say more than 100, and she plowed on with it, the Cabinet and/or Tory backbenchers would probably take matters into their hands."

Some analysts see Dec. 11 as an opportunity for a protest vote and something of a practice run before things get really serious. "Don't count May out yet," wrote Kallum Pickering, a senior economist at Berenberg bank in London, who said that while many members of Parliament dislike her Brexit plan, they have several different reasons.

"The deal is therefore not likely to pass through Parliament initially, at least after just one vote," he said.

In any event, an elaborate game of expectation management seems to be underway, with some of May's allies quietly encouraging the idea that she faces a heavy defeat in the hope that a lesser one will seem an achievement. On Monday, the government denied reports in the Sun newspaper that the vote might even be canceled to avoid a calamitous defeat.

Nevertheless, many Conservative lawmakers hate her deal and in particular its backstop plan to keep the Irish border open. That would keep the United Kingdom in a customs union with the E.U., while Northern Ireland would obey even more of the bloc's economic rules.

To critics, this seems the worst of all worlds, leaving Britain neither in nor fully out of the European Union, with no say in its rule-making and without a clear exit.