It has been a cold spring for the forces militating against a Donald J. Trump presidency.

The closer the real estate mogul draws to the Republican Party nomination, the fewer the remaining sources of solace. But one consistent theme has been the notion that the primary and the general election are as different as night and day.

“I could be wrong, but I’d be willing to make a pretty major bet that Trump’s not going to win,” Mike Murphy, a veteran Republican political strategist, said on MSNBC last week.

About 28 million people will vote in the Republican primaries, Mr. Murphy said in dismissing the “winning hype” surrounding Mr. Trump, compared with the 125 million expected to cast ballots in the general election.

“He’s entering a whole different world of voter demography,” he said.

It may be worth noting that Mr. Murphy has long had ties to the Republican establishment, which has struggled to come to terms with Mr. Trump’s ascendancy. His last “major bet” involved the failed candidacy of Jeb Bush, the former Florida governor, and he ran the $100-million-plus “super PAC” that supported his run.