The first debate between President Trump and Joseph R. Biden Jr. will begin at 9 p.m. Eastern on Tuesday and run for 90 minutes without commercial interruptions. The Times will livestream the event, accompanied by analysis and fact-checking from our reporters. The debate will also be carried on channels including ABC, CBS, CNN, C-SPAN, Fox News, MSNBC and NBC. Chris Wallace, the anchor of “Fox News Sunday,” will moderate the debate. He played that role in one of the 2016 debates between Mr. Trump and Hillary Clinton. The moderator chooses the debate topics. For Tuesday night, Mr. Wallace chose Mr. Trump’s and Mr. Biden’s records, the Supreme Court, the coronavirus pandemic, the economy, race and violence in cities and the integrity of the election. There will be 15 minutes to discuss each topic. For Trump and Biden, the debate comes with different incentives. Mr. Trump and Mr. Biden will walk onto the first presidential debate stage of the 2020 general election with a very different set of political incentives. For Mr. Trump, it is a much-welcomed chance to shake up a race in which he is currently behind. For Mr. Biden, the debate is a risky but necessary step, a close encounter with an unorthodox rival who can and will say almost anything. After complaining for months about Mr. Biden’s “basement” strategy, the debate is Mr. Trump’s biggest opportunity to reframe the election as a choice between two competing visions. The Biden campaign continues to cast the race chiefly as a referendum on Mr. Trump’s failures in responding to the coronavirus pandemic. Two things can be true at once about the stakes of the debate. First, the presidential race so far has been an extremely stable affair, with little disrupting Mr. Biden’s consistent polling lead — not a pandemic, not record joblessness, not mass protests over policing and racism, and not an unexpected Supreme Court vacancy. A 90-minute debate will be hard-pressed to move the needle more than those factors. Second, the debate still represents one of Mr. Trump’s best opportunities to jostle the current dynamic, his first chance to speak directly to an audience of tens of millions of Americans alongside Mr. Biden. How quickly does it go off the rails? Mr. Trump has always been a showman, and debates have been some of his biggest stages as a politician. He jawbones, interrupts and lashes out in unusually personal ways, and he generally exerts an intense gravitational pull toward whatever he wants the spectacle to be about. He is all but certain to attack Mr. Biden. It’s also possible he will go after the moderator, Mr. Wallace, whom the president has repeatedly compared unfavorably to his father, the former TV correspondent Mike Wallace. What past campaigns have shown is that the first half of the first debate often sets the tone — and the tone of news coverage. Mr. Biden has been pretty clear that he believes that Mrs. Clinton erred four years ago in her debates with Mr. Trump by getting into a back-and-forth argument about character. “She did what every other candidate probably would have done,” Mr. Biden said in January. The resulting debate was an ugly spectacle and, he said, “it all went down the drain.” Mr. Biden wants to avoid that — and he has been stress-tested by advisers not to respond to Mr. Trump’s obvious provocations if they are not central to his own message. Sign up for On Politics to get the latest election and politics news and insights. “I hope I don’t get baited into getting into a brawl,” Mr. Biden said this month. One wild card is how Mr. Trump’s tactics and antics will play — and how the president, who feeds off the feedback of a crowd, will respond in a debate hall without a large audience. Biden could benefit from greatly diminished expectations. For months, Mr. Trump and his surrogates have distributed unflattering and sometimes manipulative clips of Mr. Biden pausing awkwardly, stumbling verbally or just looking lost. It has been part of a concerted campaign to insinuate — and sometimes say aloud — that Mr. Biden’s mental faculties are too diminished for him to serve as president. This is not typically how expectations-setting works. Mr. Trump has lowered the bar so far — even demanding Mr. Biden take some kind of drug test — that his supporters are primed to expect a blowout on Tuesday. But Mr. Biden, even if he meandered onstage at times, ultimately won his party’s nomination after navigating 11 primary debates. Mr. Biden was memorably knocked off guard by the rival who would become his running mate, Senator Kamala Harris of California, when she attacked him for his opposition to busing decades ago. Mr. Trump has repeatedly benefited in news coverage from an expectations gap of his own: Whenever he tones down his bombast — however fleetingly — some praise tends to come in for a new tone. “Any time he utters a complete and calm sentence, people fall over themselves to call him presidential,” said an exasperated Lis Smith, a Democratic strategist who helped run Pete Buttigieg’s debate preparations during the 2020 primary. Can Trump get under Biden’s skin? Will Mr. Trump successfully goad Mr. Biden into losing his temper? Or will Mr. Biden be able to avoid walking into the trap? Democratic and Republican strategists who have gone up against Mr. Biden in debates have long identified this as a weakness, though there is more evidence of this on the campaign trail, where he has teed off on the occasional voter, than on the debate stage. When provoked, Mr. Biden is prone to getting rattled and angry and to losing his train of thought, and he risks coming across as condescending. “If Trump gets under his skin and Biden starts to do that preachy thing — ‘Let me tell you what this is,’” said Mark Wallace, who helped prepare Sarah Palin for her vice-presidential debate with Mr. Biden in 2008. “That just doesn’t fit the time.” Updated Sept. 29, 2020, 5:42 p.m. ET Mr. Trump has talked with aides about attacking Mr. Biden’s family, in particular his son Hunter Biden, and about raising the unproven sexual misconduct allegations against Mr. Biden by a former aide in the Senate, Tara Reade. For those wondering how far Mr. Trump might go down this road, it’s worth taking a look at when he debated Mrs. Clinton. Pressed on allegations of his own sexual misconduct — this was right after the “Access Hollywood” recording was released capturing him making vulgar remarks about groping women — he simply turned the spotlight to Mrs. Clinton’s husband. “There’s never been anybody in the history of politics in this nation that’s been so abusive to women,” he said. The empathy gap With more than 200,000 people in the United States dead from the coronavirus pandemic, the suffering that Mr. Biden has endured in his own life — and his ability to empathize with Americans struggling with grief now — is seen by campaign officials as one of the characteristics that most help the former vice president in this unusual year. (For those who don’t know, his first wife and infant daughter died in a car crash nearly 50 years ago. His two sons survived but one of them, Beau, died of cancer in 2015.) How Mr. Biden demonstrates that empathy — which was such a buzzword during the Democratic convention that Republicans tried to testify to a hidden softer side of Mr. Trump the next week — will be one of the ways he tries to connect with not just the Democratic base but also critical swing voters. Frank Luntz, the Republican pollster and focus-group guru, said Mr. Biden’s empathy could prove critically important, and called him “a guy who empathizes with everyone.” “The only analogy I can think of is, Joe Biden would hold a funeral for a squirrel he hit on the highway,” Mr. Luntz said. He knows firsthand. Mr. Luntz said he ran into Mr. Biden in Iowa in January, not long after the pollster had suffered a stroke, which Mr. Biden had been briefed on. “He gave me a hug and he didn’t let go,” Mr. Luntz said, “and it was really nice.” Truth, lies and fact-checking In 2016, Mr. Trump was relentless at the debates in his attacks and claims, many of them false or at least inaccurate. Mrs. Clinton tried to respond by urging viewers to go look at the fact-checking feature on her website. That proved ineffective. So how much time will Mr. Biden — or, for that matter, Mr. Wallace of Fox News — spend correcting Mr. Trump if he says things that are untrue? Mr. Wallace said ahead of the debate that a successful night would make him “as invisible as possible” for viewers, hardly a preview of an aggressive plan to fact-check the president. “If I’ve done my job right, at the end of the night people will say, ‘That was a great debate — who was the moderator?’” Mr. Wallace said on Fox News. If Mr. Biden tries to push back every time he thinks Mr. Trump says something that is false or distorted, he might find himself spending the whole night playing on the president’s turf. If he ignores him, he will no doubt face a chorus of morning-after critics wondering why he let the president get away with it. His advisers said before the debate that Mr. Biden planned to avoid soaking up his own time trying to fact-check Mr. Trump, but some key topics — such as the Trump administration’s backing of a lawsuit to overturn the entire Affordable Care Act — are more likely to elicit a factual retort. Mr. Biden has over the years proved adept at picking his debate moments — and using a smile and a laugh to dismiss an accusation. That worked with Sarah Palin in the 2008 vice-presidential debate and, to a certain extent, with Senator Bernie Sanders in the final, two-person Democratic debate this year. Whether it works with a candidate like Mr. Trump is an entirely different question. If there are fireworks, will it even matter? Many voters say the debates won’t matter. Only 3 percent of voters in a recent Monmouth University poll said that the debate was “very likely” to help determine their vote, compared with 87 percent who said it was “not likely” to affect their choice. Of course, voters also tend to say that negative ads don’t work, and yet campaigns keep airing them because history shows they do. Ms. Smith, the Democratic strategist who worked for Mr. Buttigieg, has for four years compared debating Mr. Trump to squaring up against “a chimpanzee with a machine gun”: He’s both dangerous and “completely unpredictable,” she said. She noted that Mr. Trump was widely deemed to have lost the 2016 debates — and he won the presidency anyway. As for the 2020 primaries, “I don’t think anyone thought Joe Biden was a big winner in any of those primary debates,” she said. “It didn’t matter.” The fall debate season comes after months of stability in the 2020 race, despite incredible upheaval in the nation. “Despite a global pandemic, despite an economic calamity, despite these seismic civil rights protests, nothing has changed,” Ms. Smith said. “If people have lost their job, lost their ability to go outside, can’t send their kids to school, then what is a one-hour televised debate of political talking points going to matter to them?” Katie Glueck contributed reporting.