“Even back in the summer, when he was somewhat defying gravity, somewhat defying conventional wisdom, it seemed to me there would be a moment when reality sets in,” said Rob Stutzman, a Republican political strategist who is based in California. “He would not leave himself to have his destiny settled by actual voters going to the polls or the caucuses.”

Mr. Stutzman was skeptical that Mr. Trump would be willing to endure the grind of a campaign needed to amass enough delegates to make him a factor at the Republican convention in July. That could mean a long slog accruing delegates in states where he may have to be content with third- or fourth-place performances — showings that could undercut the hyperconfident aura Mr. Trump cultivates.

Other Republican candidates are now signaling less fear of offending Mr. Trump than in the past. Senator Ted Cruz, who has treated his rival gingerly in the hope of getting Mr. Trump’s backers if he were to fade, openly mused Thursday about Mr. Trump’s ultimate political demise in an interview with a WABC Radio host, Rita Cosby.

“I don’t believe Donald is going to be the nominee,” Mr. Cruz said, “and I think, in time, the lion’s share of his supporters end up with us.”

Stuart Stevens, who was the chief strategist to Mitt Romney in the 2012 presidential race, also doubts that Mr. Trump will stay in over the long haul. “Trump’s the only person that pre-spun his exit — it’s rather remarkable,” Mr. Stevens said.

He pointed to one of the issues that has nagged the Trump candidacy from the outset — how much he is willing to spend on the race, particularly if his polling numbers start to sag. “I think we would all say this is a more serious endeavor if he was spending $2 million a week out of his own pocket, and I think it’s another sign that he’s not in this to win,” Mr. Stevens said.