Republicans who once worried that Mr. Trump might gain overwhelming momentum in the primaries are now becoming preoccupied with a different grim prospect: that Mr. Trump might become a kind of zombie candidate — damaged beyond the point of repair, but too late for any of his rivals to stop him.

Should Mr. Trump lurch into the convention so fatally compromised with both general-election voters and a sizable faction of Republicans, it could make it easier for the party to wrest the nomination away from him. But it would also make the consequences of failing to defeat him all the more ruinous if the specter of choosing a seemingly unelectable nominee does not deter Mr. Trump’s supporters.

Newt Gingrich, the former House speaker who has frequently praised Mr. Trump’s insurgent campaign, said the front-runner had made a series of bewildering and irrational mistakes. Mr. Trump’s campaign, he said, had failed to evolve beyond the “personal gunslinger, random-behavior model” characterized by the candidate.

“None of the mistakes have been forced and nobody forced him to react negatively,” Mr. Gingrich said. “It’s almost as though he is so full of himself that he can’t slow down and recognize that being president of the United States is a team sport that requires a stable personality, that allows other people to help him.”