A single ill-advised answer could turn off a crucial demographic in a battleground state. A subconscious gesture could go viral, undermining the candidate’s carefully constructed image. Or the debate could simply prove a missed opportunity when there’s few of them left before the Nov. 3 election.

Biden has to worry about looking confused or unsure, or even interrupting himself to note that his time is up, as he did several times during the primary debates.

For Trump, experts are watching how he conducts himself in a format that doesn’t suit his off-the-cuff style -- and to see if his boastful lack of preparation leaves him seeming ill-informed against a policy wonk like the former vice president.

Another key is how the candidates speak directly to the states that decide the election. The pandemic has severely limited in-person campaigning, adding importance to the first debate.

Trump’s campaign hopes to overcome Biden’s lead in the polls by locking down key battleground states, such as Michigan, Wisconsin and Pennsylvania, which he won in 2016 after solid debate performances against Hillary Clinton.

The debate will take place on Tuesday from 9 to 10:30 p.m. New York time, and will be moderated by Fox News’s Chris Wallace.

Here’s how to watch it for the parts that really make a difference.

Debate viewers should remember that many of the voters who will decide the election may not watch it.

They’ll likely hear about it after the fact via friends, social media memes and clips they stumble across online or on television. The decisive moments may not even be the most widely shared.

Veteran Democratic strategist Evelyn Perez-Verdia says that Biden’s response to a likely attack from Trump that he’s hiding a socialist agenda will be widely replayed on Spanish-language radio in Florida, where he is struggling to match Clinton’s numbers among state residents who fled socialist regimes in Cuba and Venezuela.

A concise answer that allays concerns could help him win back some of those voters, she said, possibly even clinching the state and with it, the presidency.

Those moments have plagued leading candidates in earlier campaigns. In a town-hall format debate in 2016, Trump hovered behind Clinton, attempting to make her look smaller even as she gave sharp answers to questions.

President George H.W. Bush looked at his watch during a debate with Bill Clinton and independent Ross Perot in 1992, which made him look like he felt he was wasting his time. And Ronald Reagan skewered Jimmy Carter in 1980 by asking voters in the middle of an economic downturn whether they felt better off than they did four years ago.

The most famous of all may now be considered quaint given the broadsides Trump unleashes on Twitter almost daily. But in 1988, Republican vice presidential nominee Dan Quayle compared himself favorably to John F. Kennedy. His Democratic opponent, Lloyd Bentsen, retorted to lengthy cheers and applause, “Senator, I served with Jack Kennedy. I knew Jack Kennedy. Jack Kennedy was a friend of mine. Senator, you’re no Jack Kennedy.”

Nevertheless, Quayle and Bush won the election.

Biden himself survived such a moment in the primary debates when Senator Kamala Harris, now his running mate, linked him to segregationist senators over his opposition to federal funding to bus Black children to majority White school districts, linking it to her own experience as a Black and Indian-American child in Oakland, California.

The formal topics for the debate are the candidates’ records, the coronavirus pandemic, the Supreme Court, race and violence in cities, the economy and election integrity, according to a list of debate topics announced by the Commission on Presidential Debates, the non-partisan entity that arranges these events every four years.

But the biggest question each will face is unspoken.

For Trump, the test is maintaining the right amount of composure. The president has clashed with advisers who give him bad news, given most of his extended interviews in office to friendlier news programs like “Fox and Friends” and has said he is not formally preparing for the debate.