Told of Mr. Trump’s noncommittal comment, Stuart Stevens, a senior adviser to Mitt Romney in 2012 who has become one of Mr. Trump’s most vocal critics, said that Mr. Trump was “a con man who is shocked his con hasn’t been called” and that he was looking for an emergency exit.

“He has no sense of how to govern,” Mr. Stevens said. “He can’t even put together a campaign.”

Even Mr. Trump’s supporters acknowledge that his past campaigns had the air of a vanity tour. That impression lingers. A recent Trump news release promising “a speech regarding the election” prompted many reporters and political fortunetellers to predict a declaration of his departure. But just the fact that a routine news release prompted paroxysms of conjecture throughout the political universe suggested that, as Mr. Trump might say, “there’s something going on.”

Mr. Trump’s campaign and his supporters dismiss the talk as the fantasizing of frightened liberals or frustrated establishment figures.

“He’s not going to pull out,” said Thomas Barrack Jr., a financier and real estate investor who is a close friend of Mr. Trump’s. He compared Mr. Trump’s candidacy to an innovative start-up company: “You never see disruption when it’s happening.”

In Mr. Trump’s case, the disruption is everywhere. Last fall, he said in television interviews that if his standing collapsed in the Republican primary polls, he could very well return to his business. In mid-June, amid an onslaught of negative news coverage, he joked to a crowd that he would consider leaving the race for $5 billion.

On the off chance he actually is planning to back out, what would happen?

Alexander Keyssar, a historian at Harvard who is working on a book about the Electoral College, said the process of succession would depend on “the precise moment at which he said, ‘Nah, never mind.’”

The party representatives who make up the Electoral College would suddenly have real power rather than a rubber stamp. If Mr. Trump bowed out after winning on Nov. 8 but before the electors met in each state to cast their ballots on Dec. 19, then the electors could have the opportunity to vote for another candidate, Professor Keyssar said.