It's a hallmark of the American electoral process: the presidential debate.

The first nationally televised debate in history featured John Kennedy and Richard Nixon. The visuals did not play in Nixon's favor, as he was seen sweating and looking pale after previously being hospitalized for an infection, while Kennedy looked young and gave a more vigorous performance.

More than 20 years later, Ronald Reagan, the oldest sitting president at the time, shut down some questions about his age during his debate with Democrat Walter Mondale.

In 1992's trilateral debate, President George H.W. Bush was criticized for looking down at his watch. In 2000, Al Gore was mocked for his audible sighs during his debate with George W. Bush.

In the 2020 Democratic primary, one highlight was Harris' challenge to Joe Biden on the issue of segregation and busing, which prompted her viral quote, "That little girl was me."

Most recently, the debate between President Biden and Trump changed the course of the 2024 campaign after Biden's poor performance exacerbated Democratic and voter concerns about his age and fitness to serve another four years. He announced he was exiting the race 24 days later.

Expectations matter in debates. For example, if a candidate goes in with low expectations from the public and turns in a mediocre performance, it probably wouldn't hurt them as much as if they had gone in with high expectations from the public and turned in the same performance.

Because of this, we were interested in whether Americans had higher expectations of Harris or Trump tonight — so we included this as a question in the most recent ABC News/Ipsos poll. According to that poll, 43% of Americans are expecting Harris to win tonight, while 37% are expecting Trump to win. However, 18% said neither would win or it would be a tie.

—538's Nathaniel Rakich

Tonight's debate marks a rare historical moment in that it pits against each other two candidates with significant track records in the White House.

That "political baggage" can be a curse for the candidates, according to experts who've studied debates, but also a blessing, depending on how it's portrayed and defended.

Both Harris and Trump will need to walk a fine line between affirming what they consider their successes during their administrations, experts said.

Read more here.

-ABC News' Ivan Pereira

Debates are a rare opportunity for candidates to change their perception among the American people. And right now, Harris is perceived more positively than Trump. According to 538's polling average, 46 percent of Americans view her favorably, while 47 percent view her unfavorably. While that's not exactly popular, it's a remarkable reversal from just a couple months ago. Before Biden dropped out of the presidential race, Harris had a net favorability rating of -16 points. But since she kicked off her campaign, her favorable rating has shot up, while her unfavorable rating has plummeted.

By contrast, according to 538's polling average, 43% of Americans view Trump favorably, while 53% view him unfavorably. Those numbers have been pretty consistent over the years. Trump's unfavorable rating has been about 10-15 points higher than his favorable rating since he left office in 2021, and his average job approval rating was in the same range for most of his term as president.

—538's Nathaniel Rakich