That emphasis on Mr. Trump — and brushoff of his opponents — is the right approach for Mr. Biden right now, many political veterans say. Whether he can maintain that strategy is another question, as other candidates intensify their attacks on him, and some gain ground in key early-voting states like Iowa. The first debates, too, will place Mr. Biden, the former vice president, in the same arena as his rivals while providing them the chance to pressure him on his record of relatively moderate political stances.

“The more time he’s explaining his record, the more trouble he’s going to get into,” said Jim Hodges, the Democratic former governor of South Carolina, calling Mr. Biden’s approach typical for a “front-runner.”

“The more time he’s comparing the Obama-Biden administration to Trump-Pence, the better off he is. That’s a classic strategy,” Mr. Hodges said. “But I do not believe it is one he’s going to be able to sustain over a long period of time.”

But for now, that above-the-fray approach is a defining element of Mr. Biden’s campaign. Nowhere was that clearer than in Iowa last week.