This degree of isolation leaves Mrs. May cut off from the insights that she needs for Britain to avoid calamity. Furious and frustrated politicians, experts, diplomats and business leaders have been blocked, sidelined or ignored when they have tried to brief the prime minister on the catastrophic complexities of Brexit. Even those she listens to rarely hear what she thinks. They often deduce it from how she acts afterward.

By being so silent, Mrs. May has not made the case for the Brexit deal she wants. Even if she gets Brussels to agree to it, it will fail unless she can get the cabinet and members of Parliament to back it — not by secrecy and shrewd bluffs, but by full-throated advocacy and persuasion.

And that is the fatal flaw in Mrs. May’s punching-bag strategy. She needs support she has not bothered to build. Ever since taking power in 2016, she should have been telling Britain the truth: The Brexit you hoped for is undeliverable because it promised a fantasy. The Brexiteers lied to you. We cannot have all the benefits of the European Union and none of the costs. We must compromise or face disaster.

Even some of Mrs. May’s close supporters say privately that her deal cannot survive. If it reaches the House of Commons, which must approve any Brexit agreement, many Conservative Euroskeptics will rebel, and not enough of the opposition will step in to save her. There will be uproar, chaos, talk of a second referendum and fears of “no deal.”

This situation was never inevitable. It is the product of an overcautious leader’s fundamental misjudgment of her party’s politics. Mrs. May has always been stronger than she thought; she had defeated the Brexiteers to become prime minister, and none, despite all their sniping and plotting, has had the backing to replace her since. A more courageous leader would have argued publicly for the least disruptive Brexit from the start, persuading a divided country to follow her, avoiding the immense damage we have already inflicted on our businesses, our international reputation and our relationship with our alienated, exhausted European Union partners.

Mrs. May has been inadequate, but she was the best her party could agree on. “Would anyone else step up?” an insider asked. “Everyone wants her job after March,” when Britain is set to leave the European Union. “No one else wants it now.”

Jenni Russell (@jennirsl) is a columnist for The Times of London and a contributing opinion writer for The New York Times.