When he gleefully attacked primary opponents on the debate stage, they generally reacted in one of two ways. Some sputtered, looking as if they had been hit in the face with a pie. Others, most notably Senator Marco Rubio, responded in kind, and looked bad doing it.

Mrs. Clinton, who has high unfavorable ratings of her own, will have to find the right tone if Mr. Trump fires at close range the kind of gibes he has hurled from afar at her honesty, her tenure as secretary of state, her vote to authorize the use of force in Iraq — even her treatment of her husband’s infidelities and the women involved.

She will be armed with an encyclopedic knowledge of Mr. Trump’s own significant vulnerabilities but, in past debates, Mr. Trump generally has benefited from nasty, personal volleys with his opponents, so she will have to decide when to defend, when to attack and when to rise above.

Instead of engaging Mr. Trump frontally in an ugly contest of insults, the best course for Mrs. Clinton may be to navigate around him. Even as she strikes contrasts, she should take her case directly to the camera and the American people, referring to Mr. Trump without engaging him.

But what if the other Trump appears on Monday? Though it’s tough for a brawler to show restraint for 90 minutes, it might make strategic sense for him to try and strike a reassuringly thoughtful and temperate pose.

This version of Mr. Trump would create a different challenge for Mrs. Clinton: She cannot allow her opponent to use the campaign’s largest audience to rehabilitate himself with voters who doubt his fitness for the office. She must hold him accountable for his most provocative comments and his utter lack of substance.

In past debates and interviews, Mr. Trump has demonstrated an appalling, almost defiant, lack of knowledge, and these debates, with long segments devoted to specific topics, will demand depth beyond oversimplified answers. Secret plans to defeat the Islamic State, for example, will not pass muster.