If he acquits himself well, the thinking goes, Mr. Trump may try to freeze the score rather than pushing his luck with a rematch. And if he bombs, he may well decide to eliminate the risk of a second drubbing.

“He’s a skilled media personality, but if he is held to the same commander-in-chief threshold as Secretary Clinton and fact-checked by the moderator, I wouldn’t be surprised if he took his ball and went home after the first debate,” said Jonathan Kott, a spokesman for the Democratic Senators Chris Coons of Delaware and Joe Manchin III of West Virginia.

Like most issues in politics these days, the verdict is split, but not along strictly partisan lines.

“Woody Allen said 80 percent of life is showing up,” said Will Ritter, a co-founder of Poolhouse, a Republican advertising agency based in Richmond, Va. “It is also Donald Trump’s entire debate strategy. Donald Trump will do any debate that there are cameras at.”