A strong showing could help neutralize those concerns and focus voter attention on policy issues. A weak performance could dampen fundraising and push Trump further ahead in the polls.
Both men are under scrutiny over their age and stamina. Trump is 78 and prone to making outlandish statements and mangling his sentences on the campaign trail.
Biden has a stiff gait, makes verbal slip-ups and has inconsistent public appearances, sometimes veering from solidly delivered speeches to rushed, mumbled public remarks in the same day.
A recent Reuters-Ipsos poll showed that mental fitness is one of the key issues for undecided voters ahead of the Nov. 5 election, and they are more worried about Biden.
Another Reuters-Ipsos poll this year showed that 78% of respondents - including 71% of Democrats - thought Biden too old to work in government. Some 53% said the same of Trump.
Biden's age has been relentlessly targeted by political opponents and conservative media, who have used a stew of real, fake and misleadingly edited video clips aimed at portraying him as feeble and erratic.
The White House and Biden campaign have pushed back on the effort, with limited success.
Aware that this debate could be a defining moment for his campaign, Biden has confined himself to nearly a week of "debate camp" with top advisers at the Camp David presidential retreat in the mountains of western Maryland, where he will remain until departing for Atlanta on debate day.
The goal: Show U.S. voters a wise and steady leader with a record of success, in contrast to Trump's often chaotic 2017-21 presidency, and highlight differences on abortion rights, democratic norms and fiscal policy.
To do so, Biden is spending hours each day preparing while also taking his usual daily security briefings.
Many Democrats are anxious about how well Biden will do because the stakes are so high.
"It (the age issue) is obviously a challenge and an obstacle for Biden," said one Democrat close to the White House. "I think it's always about, does he project strength and confidence? I think he's just got to demonstrate that he's in command of his agenda and confident on stage, and I think he'll do that."
The debate will take place at a CNN studio in Atlanta with no audience. The men may not bring pre-written notes on stage.
John Morgan, a Florida attorney and Biden donor, said he would like to see Biden address the question of age right away during the debate.
Biden should "talk about his physical and mental fitness compared to the other guy. He should challenge Trump to a Peloton ride and let the country see who is fitter and stronger. I'm not joking," Morgan said.
PROTECTING BIDEN
Biden's team has built a campaign aimed at protecting the president, who for most of his decades in public life was a loquacious presence, back-slapping with colleagues over a long Senate career and bantering with reporters.
Aides now stroll alongside him when he walks to his helicopter as a way of shifting focus away from his slow gait.
He wears comfortable soft-soled shoes and now gets aboard Air Force One via a shorter set of stairs, to limit the possibility of a misstep in front of the cameras.
The accommodations are more aesthetic than anything, said one former White House official, who does not believe they have worked.
"All these things that they do ... maybe mitigate risk for aesthetic reasons, (but) they don't necessarily help him," the former official said. "They all add up to this over-handling that makes him look more feeble than he actually is."
Democratic critics say Biden should spend more time engaged with the public and the press to sharpen his image and allay age concerns.
His allies say age is not the issue.
"This election's not about age. Joe and the other guy are about the same age. This election is about character, wisdom and ability," first lady Jill Biden said at an event in Philadelphia on Monday.
Biden's schedule and workload remain robust.
A Boston Globe analysis of Biden's 2023 travel found it to be on a par with Barack Obama and Trump during their third years in office. Biden visited 29 different states in 2023, compared with 30 states for Trump and 31 states for Obama, it said.
So far in 2024, Biden has made stops in 21 states at least once, according to statistics compiled by former CBS News White House correspondent Mark Knoller.
Earlier this month he did back-to-back trips to Europe, attending D-Day anniversary celebrations in France and a G7 summit in Italy.
