WASHINGTON — Perhaps never have Americans been as familiar with both presidential nominees as they are with the two on the debate stage Monday night.

Neither Hillary Clinton nor Donald Trump needs to spend much time introducing themselves to voters. Instead, they will seek to use the forum to sharpen attacks, provoke and knock the other off balance.

The result could be a battle of psychology as much as policy.

“Anything is possible,” said Dr. Elizabeth Ossoff, chair of the psychology department at St. Anselm College in New Hampshire. “In the past, we might have been able to predict where the candidates were going to go in terms of their policy strengths. … But I also think they know what people have been responding to, and the moderator is going to go to some of these character issues as well.”

Here are hurdles Clinton and Trump must overcome to sway the electorate:

Oval Office occupant: Trump thrived in the freewheeling GOP primary debates, relying on his gift of timing and his willingness to insult or be vulgar.

The most infamous instance came in Florida in March when he responded to Sen. Marco Rubio's personal attacks. “He referred to my hands. If they're small, something else must be small,” a defensive Trump said. “I guarantee you there's no problem. I guarantee it.”

While those performances were memorable, they also led to criticisms that Trump lacks the temperament to serve as commander in chief.

Whether he can tone down his approach — or whether he wants to — is one of the chief questions about Monday's debate.

Trump has in recent weeks stuck closer to a script, speaking regularly with a teleprompter and hewing to a message of change while casting Clinton as the embodiment of the status quo.

If Clinton finds a more sober Trump on stage, she may be tempted to go after what have proven to be his emotional triggers, another analyst said.

“There's one area that makes him anxious, where he doesn't look comfortable, he looks different to me as a psychologist: and that is when they talk about his bankruptcies,” said Dr. Bart Rossi, a clinical psychologist, referring to Trump's business record. “He's always good at deflecting, minimizing, marginalizing everything. But not on bankruptcies.”

Clinton's likability: Clinton's ratings on favorability and trustworthiness are among the lowest for major-party nominees — save for Trump's. Still, they are barriers for her in winning the votes of independents and Republicans put off by Trump's inexperience and bombast.

Her use of a private email server while secretary of state and the investigations it spawned have been key factors weighing her down.

She also faces a challenge unique to women in seeking power, Ossoff said.

“Even though she's seen as competent, there's a sense of unease with a woman in that position,” she said. “So people may look for, at some level — a subconscious level, a way of reducing her legitimacy toward that role by seeking out some of these other attributes that people have focused on.”

‘Crooked Hillary': Trump's need to show presidential temperament may come into conflict with one of his most effective tools: boiling down a message about his opponent into a catchy put-down. “Low energy” Jeb Bush, “Little Marco” Rubio and “Lyin' Ted” Cruz led to “Crooked Hillary.”

The insult will likely feature in Monday's debate as Trump seeks to push Clinton off-balance.

“That has a tendency to place the other person immediately on the defensive, and rather than come out and better Trump on what they know, they're responding to his attack,” Ossoff said. “That puts them in a more vulnerable and weaker position in the eyes of the people watching it.”

One on one: Trump's biggest liability could be the clock. As the number of candidates in GOP debates dwindled, the time for answers expanded. Trump struggled to fill time, and he opened himself to more fact checking from his opponents and the moderators.

Democrats hope he is unlikely to last 90 minutes on stage without an outburst, particularly in the face of sustained challenge and counterarguments from a woman.

They point to his first debate last year, in which Fox News' Megyn Kelly was the first to pose a direct question about his public comments disparaging women. He bristled at what he saw as unfair treatment: “I've been very nice to you, although I could probably maybe not be, based on the way you have treated me,” he said.