Joseph R. Biden Jr. was frustrated as he tried last year to prepare for an unwieldy debate season that stuffed as many as 11 other Democratic rivals onto a single stage. At some mock sessions, he was flanked by “Elizabeth Warren,” played by Jennifer Granholm, the former governor of Michigan, and “Bernie Sanders,” portrayed by Bob Bauer, the former White House counsel, as they peppered him with progressive lines of attack.
Mr. Biden lamented privately to advisers — and occasionally in public — that it was nearly impossible to debate with such a crowd. “If you had a debate with five other people, you might actually get a chance to say something,” Mr. Biden told donors in Hollywood last fall. He would deliver more forceful performances as the field narrowed, he promised.
Now, Mr. Biden will get his chance. The former vice president will debate President Trump for the first time on Tuesday, a date circled for months as one of the most consequential on the 2020 political calendar, and one of a dwindling number of chances for Mr. Trump to chip into Mr. Biden’s lead in the polls.
Given Mr. Biden’s current polling edge, his advisers have been downplaying the debate’s significance even as the former vice president has plunged himself into days of intense preparations. He is rehearsing and studying his briefing books — Mr. Biden has long preferred the Arial typeface, 14 point — in a process overseen by his longtime adviser and former chief of staff, Ron Klain, who similarly ran Hillary Clinton’s debate camp.
“It is definitely one of the last things that could move the race,” said Jay Carney, the former White House press secretary under President Barack Obama and a former adviser to Mr. Biden. “The odds of it moving the race are not high. But there are not that many opportunities.”
The risks for Mr. Biden are manifold. Allies and people who have coached him for past debates fret about his temper and tendency toward defensiveness when it comes to his own lengthy record. His debate showings during the 2020 primary — ultimately sufficient to win — were often marked by meandering digressions and antiquated references and were rarely, if ever, hailed as command performances.
And in Mr. Trump, he faces an asymmetrical antagonist, someone who has no qualms about deploying crudity, insults, distortions and falsehoods for political advantage. The absence of guardrails is already evident. On Sunday, Mr. Trump demanded that Mr. Biden should have to take some kind of drug test before the debate.
The president, who has undertaken less formal debate prep, has mused with aides about bringing up the business dealings of Mr. Biden’s son Hunter and the sexual assault accusations of Tara Reade, which have otherwise faded from the campaign.
Mr. Trump has already engaged in months of relentless and often misleading attacks on Mr. Biden’s mental acuity that have lowered the bar for the Democrat. A strong performance in the first 45 minutes could torpedo that line of attack with many viewers, while Republicans are eager to seize on any verbal missteps to push often-distorted story lines about Mr. Biden.
Mr. Biden has repeatedly signaled his determination to avoid a repeat of 2016 and Mr. Trump’s ugly clashes with Mrs. Clinton, and he has second-guessed whether her response onstage to the “Access Hollywood’’ tape wound up sullying them both. He has cast her response as a missed opportunity to turn the subject back to her agenda.
“I hope I don’t get baited into getting into a brawl with this guy,” Mr. Biden told donors at a fund-raiser earlier this month.
The debate will represent the first joint appearance of the general election for two candidates who offer starkly different visions of the country, and whose campaigns have reflected those contrasts. Mr. Trump, 74, against the advice of public health officials, has been drawing thousands of supporters for large rallies at which he and many of his supporters do not wear masks. Mr. Biden, 77, almost always masked in public, has adhered to a more limited schedule, with white circles taped to the ground during small gatherings to delineate the appropriate social distances attendees must maintain.
That contrast could be the first visual of the debate, especially if Mr. Biden emerges wearing a mask. Mr. Biden has spent the past six months employing a play-it-safe strategy — health-wise and politically — by campaigning mostly virtually from his Delaware home and making Mr. Trump’s response to a pandemic that has now cost more than 200,000 lives the focal point of the election.
His advisers want the debate — and the race itself — to be a referendum on Mr. Trump’s stewardship of the health crisis, even as Mr. Trump’s rush to fill a Supreme Court vacancy has created a new flash point.
“The Biden campaign wants a campaign that only includes Donald Trump,” said Brad Todd, a veteran Republican strategist. “The Trump campaign wants Donald Trump vs. Joe Biden. And on debate night, the Trump campaign is going to get what it wants.”
Still, Mr. Biden’s team has provided talking points to surrogates outlining their belief that little can occur onstage that will fundamentally change the shape of the race.
David Axelrod, the former chief strategist to Mr. Obama, who attended some of Mr. Biden’s vice-presidential debate preparations, said that going up against Mr. Trump “is not like preparing for a normal debate.”
“How do you deal with serial lying?” Mr. Axelrod said. “How do you deal with the provocations. He can be exasperating. How much do you want to tangle with him on every point? How do you keep from going down rabbit holes that don’t really lead anywhere?”
Mr. Biden’s advisers do not hope to muzzle his indignation entirely, campaign officials said — but want to ensure that any temper-flashing moments are channeled in a productive direction: anger about the loss of life during the pandemic, Mr. Trump’s sometimes indifference to public health guidelines and perhaps especially the president’s comments, as reported by The Atlantic, that American soldiers who died in war were “suckers” and “losers.”
Senator Chris Coons, Democrat of Delaware and a close Biden ally who has offered “broad framing input” about the debate, said he had warned against “being surprised about the attacks and the spectacle.”
The focus for Mr. Biden, Senator Coons said, should be “bringing it back” to the concerns of the voters.
Mr. Biden has settled on a favorite pre-debate phrase about the race, calling it a contest between his hometown, Scranton, and Park Avenue, part of his effort to connect with working Americans, especially in the Industrial Midwest. Senator Coons said Mr. Biden had readied some “sharp rejoinders” to highlight “what’s at stake.”
Mr. Biden’s advisers have signaled that the former vice president does not plan to spend most of his time fact-checking Mr. Trump, and hopes the moderator, Chris Wallace of Fox News, fills that breach.
“If you take on that role, you seem small,” Mr. Carney said.
The 90-minute debate will be divided into six segments, selected by Mr. Wallace: the pandemic, the economy, the Supreme Court, the “integrity of the election,” the “Trump and Biden records” and “race and violence in our cities.” The latter has led to some Democratic objections for its framing around violence in cities — a centerpiece of Mr. Trump’s advertising over the summer — rather than around police brutality and calls for racial justice.
Mr. Biden’s team has kept the circle of those directly involved in debate preparations small, with a limited number of in-person participants.
His stable of debate advisers includes many people he has worked with for years, and in some cases decades: Mr. Klain; Steve Ricchetti, a longtime adviser; Mike Donilon, his chief strategist; Anita Dunn, a senior adviser; and Mr. Bauer, who played Mr. Sanders in the primary and has taken on the role of Mr. Trump in some practice sessions.
Other senior officials involved include Symone Sanders, Kate Bedingfield, Julie Chavez Rodriguez and two top policy advisers, Jake Sullivan and Antony J. Blinken.
Mr. Klain, who oversaw Mrs. Clinton’s debate preparations and who worked for Mr. Biden years ago during his short-lived 1988 presidential run, is “running the show,” according to one Biden insider.
Others involved in past Biden debate preparations noted that Mr. Biden does have at least some experience with an unusual opponent: Sarah Palin in 2008.
During the Democratic primary, Mr. Biden, steeped in the more civil politics of a previous era, was sometimes reluctant to lace into his opponents too vigorously. Allies expect no such inhibitions against Mr. Trump, a man whose approach to politics and power plainly offends Mr. Biden at a visceral level.
“With Democratic opponents, he was dealing with family,” said Representative Donald McEachin, Democrat of Virginia and an early Biden supporter. He predicted a more aggressive posture on Tuesday, saying Mr. Biden “won’t be afraid to throw a punch.”
Senator Chris Van Hollen of Maryland, who played Representative Paul Ryan of Wisconsin during Mr. Biden’s 2012 vice-presidential mock debate sessions, said he was not nervous about the debate partly because Mr. Trump was trying to paint the former vice president as someone he was not: a left-wing radical.
“They’re essentially ascribing to Joe Biden positions and statements that are not his,” Senator Van Hollen said. He described Mr. Biden as a “competitive” preparer who sought to master the policy minutiae — sometimes to a fault.
“He can get into the weeds,” Senator Van Hollen said.
One challenge for Mr. Biden will be to avoid relitigating the details of past controversial votes, topics he sometimes struggled to navigate onstage during the primary when pressed on issues like the Obama administration’s record on deportations or his own history of dealing with segregationist senators.
Mr. Biden sometimes digresses into distinctively senatorial phraseology — like “confiscatory,” which he used in a recent CNN town hall. At that same event, Mr. Biden summarized his preparations as “making sure I can concisely say what I’m for and what I’m going to do.”
Mr. Biden has had the occasional transcendent debate moment. In the 2008 primaries, he batted away a moderator’s question about whether he had the discipline to contain his verbosity on the world stage.
“Yes,” Mr. Biden said, before going silent with a smile.
His other most memorable moment was slamming Rudolph W. Giuliani during a vice-presidential debate in 2007 for constantly citing his time as New York City’s mayor during 9/11, saying Mr. Giuliani’s sentences had only three core elements: “a noun and a verb and 9/11.”
That line of attack could serve as a road map of sorts for Mr. Biden’s own performance as he seeks to pivot back to Mr. Trump’s handling of the pandemic and the subsequent economic fallout this year.