It will be a study in contrasts of age, gender, race, temperament and policy. It will also be the first time in U.S. presidential history that a former prosecutor will take the debate stage alongside a convicted felon with the White House at stake.
Vice President Kamala Harris, the Democratic nominee, has served as a trial lawyer, district attorney and attorney general in California. Former U.S. President Donald Trump, her Republican rival, has been convicted of 34 counts of falsifying corporate records to cover up a sex scandal.
The two will square off in Philadelphia on Tuesday night for their first and perhaps only debate, just 75 days after Joe Biden's poor performance against Trump caused a political earthquake that ultimately forced him to drop out of the White House race.
Few expect such a transformative outcome this time around. But Trump has his last chance to end Harris' extended "honeymoon," as the Democrat seeks to prosecute her opponent for the egregious liabilities of tens of millions of voters watching live on television.
"This is the first time Donald Trump has actually been questioned in front of the American people," said Tara Setmayer, former communications director for Republicans on Capitol Hill. "Kamala Harris's career and experience as a prosecutor, attorney general and senator is something that Trump cannot discount in this debate."
This will be Trump's seventh appearance in a national general election debate, making him the most experienced debater in U.S. presidential history. Against Biden in June, he repeated familiar lies that went largely unchallenged. Harris is expected to be a more formidable opponent and could put Trump on the defensive over facts, policy and his conduct after the 2020 election.
The 59-year-old has not been shy about embracing her law enforcement career on the campaign trail so far. A video at the recent Democratic national convention in Chicago declared, "That's our choice. A prosecutor or a criminal." In a speech accepting the party's nomination, Harris told cheering delegates, "Every day, in court, I stood proudly before a judge and said five words: Kamala Harris, for the people."
She has also touted her record of fighting predators and fraudsters, telling people across the country, "I know the type of Donald Trump!" Harris brought that experience to light during her memorable cross-examination of Brett Kavanaugh during the 2018 Senate confirmation hearings after then-president Trump nominated him to the Supreme Court.
But she's unlikely to take Trump directly to task over his convictions - or three other criminal cases still pending. When an audience member at a rally in New Hampshire this week shouted, "Lock him up!" Harris responded, "Well, you know what? The courts are going to handle that, and we're going to handle November. How about that?"
In May, Trump became the first former U.S. president to be convicted of a felony when a New York jury found him guilty on all 34 counts in a scheme to illegally influence the 2016 election through a hush-money payment to an adult film performer. On Friday, Judge Juan Merchan postponed Trump's sentencing until Nov. 26 - after the Nov. 5 election date.
For any other candidate on a debate stage, the convictions would be a huge burden. But Trump has repeatedly inflamed his base by falsely claiming that the case, and others involving election interference and mishandling of classified information, are bogus and politically motivated. Should the subject come up Tuesday, he will likely cast himself as a martyr, reminding viewers that he was nearly assassinated in July.
The 90-minute duel, held at the National Constitutional Center in Philadelphia, will be moderated by ABC News anchors David Muir and Linsey Davis. Under rules negotiated by both campaigns, there will be no live audience and the candidates' microphones will be muted when it is not their turn to speak.
The same rules appeared to work in Trump's favor when he faced Biden in Atlanta in June. Aaron Kall, director of debate at the University of Michigan, said: "Trump adapted well to the lack of a crowd and the turning off of the microphones in Atlanta. Biden clearly did not."
"He'd never debated without an audience; same with Harris. Not getting feedback and not knowing where things stand, you have to rely on your own judgement and who has better media instincts than a reality TV host?"
Muting the microphones could not only protect Trump from himself - he interrupted Biden 71 times during their first presidential debate in 2020 - but also prevent Harris from delivering sharp rebuttals like "I speak," a phrase she uttered to Mike Pence during the vice presidential debate four years ago.
Harris and Trump have never met in person and will likely play the roles of boxer and fighter, respectively, in the city of Rocky Balboa. Trump, 78, is not known for his discipline, preparation or commitment to the truth. His debate performance, like his governing style, has typically been based on gut feelings rather than considered analysis.
Kall, who has attended many presidential debates, added: "You can never write him off because he's all over the place and in a kind of shoot-and-shoot format and when you think you've got him on something, he quickly moves on to something else. It's hard to keep up with him, so she has to pick her spots.
"He's always been known as the most effective counterpuncher. Sometimes he doesn't throw the first volley, attack or argument, but if she decides to go on the attack as a prosecutor and treat him that way, he can react even more deadly."
Trump has struggled to find a coherent and effective line of attack on Harris since she entered the race. He has accused her of being a radical leftist while also suggesting that she is responsible for Biden's more centrist policy agenda. He has questioned her intelligence and racial identity. He has also faltered in his attempts to achieve consistency on the burning issue of abortion rights.
Republicans hope his debates will be more focused and that he will avoid overtly sexist or racist behavior. The last time he faced a female candidate, Hillary Clinton in 2016, he physically hovered behind her in a debate and called her "the devil" and a "nasty woman."
Larry Jacobs, director of the Center for the Study of Politics and Government at the University of Minnesota, said: " The Harris campaign was desperate for his mic to be live because they think he's his own worst enemy and that's true. He has a very short attention span.
"He's a remarkably undisciplined candidate, particularly at this level, and he's very uncomfortable with women and people of color. I don't see that changing. He's made some pretty offensive comments about Kamala Harris already in this campaign. I would expect more of that, and it's possible that Kamala Harris is going to push him in ways that could provoke that reaction."
Harris enters the debate with momentum. After she closed the convention on Aug. 22, her campaign announced that she had raised more than $500 million since entering the race. The polling aggregator website 538 shows Harris ahead by three percentage points in national polls, but the races in some swing states are much tighter.
Simon Rosenberg, a Democratic strategist, said: "Kamala Harris is a capable politician operating at the height of her powers. She's going to have a good debate and he's going to, in all likelihood based on his current string of public appearances, say things that are ugly and shocking and he's going to do further damage to an already damaged campaign."
But the vice president may come under pressure to explain her about-face on issues like universal health care, fracking, plastic straws and decriminalization of illegal border crossings. She could face questions about the Biden-Harris administration's economic performance, particularly inflation, forcing her to defend her boss and pledge to turn a new leaf.
Lanhee Chen, a fellow in American public policy studies at the Hoover Institution think tank in Stanford, California, said: "The content of what she's rolled out so far either completely contradicts her past or they're just not really good ideas for the most part. If she can actually propose some new ideas in this debate and give people some fodder for the mill, that's a much better approach."."
History, however, shows that debates are less about policy than about memorable moments. Examples include Ronald Reagan's "There you go again," Jimmy Carter's banter, George H.W. Bush's glance at his watch, Al Gore's sighs, and Trump's apparent threat to jail Clinton. Political scientists also continue to question whether the impact on public opinion is fleeting or long enough to make a difference on Election Day.
Chen, who was policy director for Mitt Romney's 2012 presidential campaign, cited as an example Romney's strong first debate performance against a lackluster President Barack Obama in 2012. "We saw a significant improvement for Romney in both the public polls and our private polls after that great debate performance against Obama in 2012," he recalled.
"In that first debate he picked up a number of points that were way out of the margin of error in many places. It was a couple of weeks of positive momentum and then the race kind of ground to a halt again."
Chen added: "The debate doesn't just happen in a vacuum. You have the debate, but you also have world events and you also have what the campaigns do after the debate. The debate will have an impact, but the impact is likely to be short-lived and will eventually wash away with other campaign events as they happen."