President Trump and former Vice President Joe Biden will meet face to face for the first time on Tuesday for a highly anticipated debate that could set the course for the five remaining weeks of the campaign.

The clash will present both candidates with an opportunity to frame the choice before voters this November, to lay out their case and prosecute the case against each other.

But it will also give both candidates the opportunity to commit gaffes that will define them in the minds of voters. In an election in which so few voters remain undecided, the prospect of a self-inflicted wound — a forgetful moment, an awkward line, a slip of the tongue — is a greater threat to either Trump or Biden than anything they might say about each other.

“A debate does not change a lot of minds. Most people come to the debate when we get to this point in a long campaign and they’re committed. They’re viewing the debate for reinforcement and motivation,” said Mitchell McKinney, director of the Political Communication Institute at the University of Missouri.

The primary goal Trump and Biden must achieve isn’t to win, debate historians and experts said. It’s to not lose.

“What you want to do is you want to hold your base, so you try not to lose, basically. You try to avoid the big gaffe, the misstatement,” said Joseph Tuman, a political communications expert at San Francisco State University.

The challenge will be especially difficult for Trump, as the incumbent. Sitting presidents have had rocky first debates in recent history, partly because they are used to being the unquestioned center of attention in every room they enter. For four years, Trump has not stood on stage with an equal.

“There is a long history of incumbent presidents struggling, especially in their first debates. They’re out of practice,” said Aaron Kall, the dean of students and director of debate at the University of Michigan. “You’re the president. You have people that work for you, and they’re unlikely to tell you what you really need to hear.”

In 2012, President Obama stumbled so badly in his first clash with Republican nominee Mitt Romney that he apologized afterward to his staff. In 2004, President George W. Bush was caught flat-footed by then-Sen. John Kerry’s (D-Mass.) aggressive attacks.

In 1984, President Reagan seemed unfocused and forgetful in his first debate against former Vice President Walter Mondale. That raised questions about Reagan’s age and mental fitness — questions he turned back with a memorable quip in their second meeting.

“I want you to know that also I will not make age an issue of this campaign,” Reagan said at the subsequent debate. “I am not going to exploit, for political purposes, my opponent’s youth and inexperience.”

Obama acknowledged to staffers he had not prepared for the first debate against Romney, and he turned in a stronger performance in his second outing. Trump has said he is doing little formal preparation, too. He is unlikely to sit for sessions in which aides pepper him with criticism to gird him for Biden’s attacks.

“In Mr. Trump, we have a person who doesn’t take criticism well,” Tuman said. “This is a guy with thin skin.”

Still, rocky debate performances are not a death knell for a sitting president. Obama, Bush and Reagan may have lost some voters in their first outings with rivals, but all three won reelection — Reagan won 49 states.

Where Trump may be out of practice standing next to a rival who will challenge him directly, Biden has much more recent experience. He struggled to stand out in many of the early debates during the Democratic primary, and he was especially taken aback by the criticism lobbed his way — most notably by Sen. Kamala Harris (D-Calif.), who is now his running mate.

But Biden grew more comfortable on the debate stage. He attacked former New York City Mayor Michael Bloomberg in the last multi-candidate forum just before Super Tuesday, and he turned in perhaps his best performance in a one-on-one match-up against Sen. Bernie Sanders (I-Vt.), just before primaries that effectively ended Sanders’s chance at the nomination.

In vice presidential debates in 2008 and 2012, Biden effectively parried critiques from Republican nominees Sarah Palin and Paul Ryan.

“He didn’t do great when there were a half dozen, 10 people on stage and there wasn’t a lot to contrast. But he did do well against Bernie Sanders, who’s a formidable debater,” Kall said. “His debate performance against Sarah Palin in 2008 and Paul Ryan in 2012 were both above average.”

Trump has helped Biden, in a way, by setting expectations of the former vice president artificially low. Trump has questioned Biden’s mental stamina, and his surrogates have wondered whether Biden would show up, though Biden’s campaign has never uttered even the suggestion that he would not participate.

Some in Trump’s orbit have belatedly changed course, pointing to Biden’s long career in the Senate and his experience on the debate stage in hopes of raising expectations. But the bar has been set, most prominently by the president himself.

“What [Trump] has done, in many ways, is lower expectations for voters, who are now watching these debates looking to see if Biden is going to confirm Mr. Trump’s characterization of him,” Tuman said. “All Biden has to do in this first debate is sound like a guy with a pulse and someone who can do subject-verb-object in sentence construction and he’ll get over the low bar Trump has set for him.”

The wild card in any one-on-one match-up will be the third man on stage, "Fox News Sunday" host Chris Wallace. Wallace has already laid out the six topics he will focus on: the candidates’ respective records, the coronavirus pandemic, the economy, race and violence in American cities, the open Supreme Court seat and the integrity of the election.

But what will remain unclear until the klieg lights are on and the cameras are rolling is how active a role Wallace wants to play.

“He’s a good moderator, he has a lot of experience. He’s not going to be intimidated by the candidates,” Kall said of Wallace, who is moderating his second general election debate. “The main question will be how much does he want to inject himself.”