The Conservatives’ leaders cannot admit to the electorate that they were deceived without splitting the party. And instead of apologizing for misleading voters, Mr. Johnson and the other Brexiteers have doubled down, taking refuge in optimistic slogans and vapid promises, refusing to believe the increasingly agitated evidence from hospitals, airlines, farmers, supermarkets and factories that a hard Brexit will damage them all.

Last week, Mrs. May finally attempted to force a recognition of reality on her divided cabinet by coming up with a compromise; a partial Brexit that allows goods free access in and out of Europe at the cost of accepting many European rules. It was an imperfect plan, but still it provided, finally, a starting point for negotiations with Brussels.

For three days, that compromise held, until the first political delusionist, the Brexit Secretary David Davis, broke free, still claiming that in some magical future Britain could get almost everything it wanted, if only the country would just stand by its demands.

Petrified of being outflanked, Mr. Johnson followed suit, bringing with him the implicit threat that he could lead a rebellion against the government that other hard-line Brexiteers will follow. It is a desperate move by a man who has lost almost all the credibility he had three years ago.

All of Mr. Johnson’s weaknesses have been exposed: his lazy reluctance to do detail, his preference for bluster over thinking, his contempt for business. The campaign was meant to secure his future; instead, in damaging the country, he fears he has wrecked his own future, too. As one of his allies told me last month: “He knows that the verdict of history is about to come down on him — and bury him.”

Mr. Johnson seems to believe that this is his last chance to become prime minister: After his resignation this week, he hopes to be reborn as a rebel who will lead the party. But more likely is that he will once again create political chaos without delivering what he wants.

Two years ago, the side effect of Mr. Johnson’s ambitious maneuvering was to split the country and risk the prosperity and security of all Britons for decades. Now, just as a fragile basis for negotiation emerges, his selfish drive for vindication, attention and admiration threatens that, too.