Age and mental sharpness have been at the center of the contest between the Republican Trump and his Democratic rival, President Joe Biden, often getting more attention than substantive policy issues in the run-up to the Nov. 5 election.

Public opinion polls show Americans are more worried about the advanced age of Biden, who is 81. But at 78, Trump is just three and a half years younger, and he would be the oldest person ever to be inaugurated if he wins a second term.

Trump is scheduled to speak on Friday at a birthday party organized for him in West Palm Beach, Florida, by a group of die-hard supporters.

On the campaign trail, Trump has not explicitly made an issue of Biden's age, but has sought to capitalize on his opponent's every verbal mistake, as well as Biden's slowing gait, to cast him as unfit for the Oval Office.

Biden has responded to questions about his age by telling voters to focus on his accomplishments in office as evidence of his acuity and strength. He has also described Trump as a threat to democracy and criticized his sometimes rambling speeches, as well as Trump's use of inflammatory rhetoric against immigrants.

Still, even some Democrats have expressed concerns about Biden's ability to complete another term, which would take him to age 86.

In a Reuters/Ipsos poll in February, some 78% of respondents - including 71% of Democrats - said Biden, already the oldest president, was too old to work in government. Some 53% of respondents said Trump, who was president from 2017-2021, was too old for government work.

"It's not about age, it's about mental competence," said Trump campaign spokesperson Karoline Leavitt, arguing that voters can see the contrast between Biden and Trump, whom she described as "sharp as a tack with elite stamina."

The Biden campaign did not respond to a request for comment.

Presidential historian Timothy Naftali said Trump projects energy in public appearances, making him appear to be more vital physically, but that does not mean he is sharper mentally.

"It's not clear listening to the two men who's in better command of his faculties," Naftali said.

Allan Lichtman, a history professor at American University and well-known presidential prognosticator, said Trump had made gaffes and spread false information to an extent that should be raising more questions about his mental fitness.

"People somehow focus on the mistakes Biden has made while totally ignoring the way in which Trump seems to be entirely unhinged from reality," he said.

Trump and Biden are neck-and-neck in national opinion polls, with Trump ahead in several of the battleground states that could decide November's contest.

It is unclear just how much age will be a factor in the final outcome. Among issues that voters will be weighing is the strength of the economy, which overall is performing well but is beset by inflation, as well as immigration and abortion rights.

Voters also have Trump's legal troubles to consider. Last month, a New York jury found him guilty of falsifying business records to cover up a payment to a porn star on the eve of the 2016 election. He faces three additional criminal cases, though none are likely to go to trial before the election.

The first televised debate on June 27 will be an important test for both Biden and Trump, with voters looking for verbal slip-ups as a possible indicator that they might not be up to the task of leading the country.