“The deal is therefore not likely to pass through Parliament initially, at least after just one vote,” he said. “There is so much debate about a possible second vote now that many MPs probably think they can vent their frustration on the first go before getting more serious the second time around.”

In any event, an elaborate game of expectation management seems to be underway, with some of Mrs. 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 so-called backstop plan to keep the Irish border open. That would keep the United Kingdom in a customs union with the European Union, 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.

On Monday, critics from several parties denounced the government’s refusal to publish the full legal advice it received on the Brexit plan, despite an agreement from lawmakers on the need to do so.

In some rare good news for Mrs. May, one prominent Brexit supporter, Michael Gove, the environment secretary, has fallen in behind her plan, after a period of silence during which, according to news reports, he considered resigning from the cabinet and turned down the position of Brexit secretary.