Joe Biden has the scouting report of the president’s past debates and four years in office. One way to use it? Respond to unpredictability with a simple, “Here’s my stability plan.”

Hillary Clinton was attempting to talk about health care in the second presidential debate in 2016 when her opponent, Donald J. Trump, suddenly moved across the stage and lurked behind her, in the manner of Snoopy pretending to be a vulture in the Peanuts cartoons. She could feel him breathing down her neck.

“It was incredibly uncomfortable,” Mrs. Clinton wrote in her memoir “What Happened.” “It was one of those moments where you wish you could hit pause and ask everyone watching, ‘Well, what would you do?’”

Americans had never before confronted a serious presidential candidate as aggressively unpredictable as Mr. Trump, and at the time his opponents had not yet developed a counterattack. But now another candidate, Joseph R. Biden Jr., is about to face Mr. Trump in a debate. He has had four years to observe the president’s techniques.

The stakes are high. The mood is feral. And the question for Mr. Biden is: What is the best way to confront an opponent who does not play by the rules?

“It’s like the Vietnam War,” Elinor Greenberg, a therapist in Florida and author of “Borderline, Narcissist and Schizoid Adaptations,” said of engaging with someone like Mr. Trump. “You’re wearing a costume and expecting to face your opponent across the battlefield, but suddenly you find yourself in the jungle and you don’t have a set of rules for what you’re dealing with.”

The question is as much psychological as political. Although American mental health professionals tend to follow the Goldwater Rule, which holds that they should not offer formal opinions about public figures they have not examined, a number of psychologists have broken with that tradition in the Trump era, saying their duty to warn the public supersedes their professional reserve.

In the 2017 book “The Dangerous Case of Donald Trump,” 27 mental health experts argued that the president’s mental instability made him unfit for the job and a threat to the country. He exhibits signs of narcissism, sociopathy and extreme present hedonism, they wrote; he is grandiose, unable to accept criticism, vain, impulsive, lacking in empathy and desperate for affirmation.

How should Joseph R. Biden Jr. debate such a president? Saying they were alarmed at Mr. Trump’s behavior, a number of mental health professionals offered a variety of suggestions for how to respond.

Martha Beck, an author and sociologist, said that Mr. Trump was like “a tantrummy child” in a supermarket.

“If the mom lies down and tantrums beside him, things don’t get better, and if she just goes on shopping, things don’t get better either,” she said. “Someone has to say, ‘Stop, this is unacceptable.’”

She recalled the moment in 2016 when Senator Marco Rubio, one of Mr. Trump’s opponents in the Republican primary, tried to descend to his level by mocking the size of his hands and saying that “he’s not going to make America great; he’s going to make America orange.”

Such tactics, she said, are wasted on the president. “If Rubio had said, ‘Stop it, everybody in this room can see what a conceited, pompous bag of wind you are, we had our own abusive fathers, and we don’t need you’ — that would have been effective,” she said.

There was also the moment in the Republican primary campaign when Megyn Kelly of Fox News challenged Mr. Trump by reciting a list of disparaging comments he had made about women. Rather than defending his record or saying he had learned his lesson, Mr. Trump attacked “political correctness,” sneered at the talk show host Rosie O’Donnell and said that although he had been “very nice” to Ms. Kelly, “I could probably not be, based on the way you have treated me.” He then launched into a bad-tempered tirade against Ms. Kelly on Twitter and television.

It might be worth keeping that response in mind if anyone at the debate brings up the subject of the president’s business acumen and financial probity, following The Times’s report about the precariousness of his financial situation and the revelation that in both 2016 and 2017, he paid just $750 in federal income taxes.

“One of the classic things with a narcissist is that if you criticize or dismiss or belittle them, they’re likely to get aggressive or defensive,” said W. Keith Campbell, a professor of psychology at the University of Georgia. His advice for Mr. Biden, he said, would be to lower the drama, emphasizing that the president has destabilized the country but that Mr. Biden would “make things sane again.”

“The reason that someone like Trump won the presidency and may win again is that when things are unstable and uncertain, people want strong narcissistic leaders,” Dr. Campbell said. “So you say, ‘Here’s my stability plan.’”

Dorothy L. Espelage, a professor of education at the University of North Carolina at Chapel Hill whose research focuses on adolescent bullying, said Mr. Trump is similar to the sort of “Machiavellian bully” who exhibits “heightened narcissism and severe insecurity” and who targets weaker children in the playground.

But if you have to engage, she said, it’s important to keep hold of your emotions and recognize that your opponent will never admit to being wrong. So you have to deploy cool dispassion while challenging the behavior.

“I study students with disabilities,” she said, and she still remembers with horror the moment in the 2016 campaign when Mr. Trump mocked a disabled reporter. “I would say to him, ‘What is your point? Enough is enough. What’s your point, President Trump? It sounds like you’re really getting yourself worked up.’”

Jay Carney, a former press secretary for President Barack Obama who worked as a spokesman for Vice President Biden and is now the senior vice president for global corporate affairs at Amazon, said that Mr. Biden should avoid getting into “an insult exchange” with the president.

“The fact that the president likes to insult his opponents, or his critics, or people in general, is not new information to voters, and it’s not going to move them,” Mr. Carney said. “I think Biden’s task will be to quickly divert to something that matters to voters” — the coronavirus, the economy, health insurance.

What about when he exaggerates, distorts the truth, makes false accusations, lies? Should Mr. Biden ignore him, or double down?

Mary L. Trump, the daughter of Mr. Trump’s late brother, Fred, and the author of “Too Much and Never Enough,” a psychological indictment of the president, said it was essential to push back.

“Donald needs to be fact-checked in real time,” she said. “By failing to stop him in mid-lie, the lie kind of settles, and then when it gets countered it becomes two different opinions about something, as opposed to, ‘This is a lie and it should not be allowed to continue.’”

Tony Schwartz, the journalist who wrote Mr. Trump’s book “Trump: The Art of The Deal” more than 30 years ago and later denounced the president as a dangerous sociopath, said that Mr. Biden should needle Mr. Trump by unleashing a barrage of evidence of the president’s failings and interrupting him when he tries to “flood the zone” with irrelevant commentary.

“In a very calm, unequivocal way, he ought to be saying things to Trump about Trump that we know to be true. ‘We know you don’t read books. We know you can’t sit still long enough to be briefed. We know you spent x hours on the golf course this year,’” Mr. Schwartz said. “The more specific the comment the better. ‘You allowed thousands of people to come to rallies without wearing masks. You’ve told x number of lies, and you’ve already told eight tonight.’”

When Mr. Trump tries to obfuscate by attacking back, said Jennifer Contarino Panning, the Illinois psychologist who coined the term “Trump anxiety disorder” to describe the distress Mr. Trump causes in many Americans opposed to his presidency, Mr. Biden should not take the bait.

“Biden needs to come across as forceful and confident to match Trump’s bluster,” she said. “We can be certain that Trump will use manipulative strategies to try and throw Biden off, so Biden should be prepared to remain calm and direct any discussion of his lies to the American people, not to Trump.”

The best way to get under the president’s skin, she said, is to “appeal to Trump’s emotions and insecurities.”

“Trump hates being laughed at or mocked, so Biden utilizing these techniques would be useful to throw Trump off,” she said. “Biden should be prepared for a backlash when using these techniques. And it would behoove him to remain confident and strong in his assertions.”

Robert Jay Lifton, a lecturer in psychology at Columbia University and a contributor to “The Dangerous Case of Donald Trump,” said he agreed with Michelle Obama’s philosophy from the previous election: “When they go low, we go high.”

“Going high in this conversation is going with factual truth,” he said, “and at the same time going with an ethical baseline in connection with that truth — and confronting its absence in him. Trump is very skillful at all this, but he’s not superhuman.”

Returning to Mrs. Clinton and the problem of the menacing debate opponent — and the notion of esprit d’escalier (the thing you wish you had said at the time but did not think of until later) — what should she have done?

“Forget whether it’s logical,” Dr. Greenberg, the Florida therapist, said. “He’s making a nonverbal statement that he can invade your space. And whoever is against him in that situation has to think of a novel response.”

Or, as Mrs. Clinton wrote: “Do you stay calm, keep smiling and carry on as if he weren’t repeatedly invading your space? Or do you turn, look him in the eye and say loudly and clearly, ‘Back up, you creep. Get away from me.’”

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News – Engage or Proceed? Debating Trump, the Unpredictable Aggressor