WASHINGTON--For 90 minutes on Thursday night, U.S. President Joe Biden has the chance to show doubting Americans that, at age 81, he is capable of handling the rigors of another four-year term, when he faces off in a debate against Republican Donald Trump. 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.