President Joe Biden is hunkered down with aides at Camp David for several days to get ready to debate rival Donald Trump, who is eschewing traditional preparation and instead holding informal policy discussions between campaign stops.

The face-off in Atlanta, at 9 p.m. ET on Thursday (0100 GMT on Friday), will be the earliest presidential debate in modern U.S. history and a critical event for both candidates.

Biden, 81, and former president Trump, 78, are neck-and-neck in national opinion polls, with a considerable slice of the electorate still undecided five months before the Nov. 5 vote.

The debate will provide the starkest contrast yet of the two men, the oldest candidates ever to seek the U.S. presidency, as voters question their age and mental sharpness.

"It's an incredible test of their cognitive competence," said Patrick Stewart, a political science professor at the University of Arkansas who has written a book on presidential debates. "This is our chance to see how much they've declined or if they've declined."

With strict speaking limitations, a ban on notes and no audience to cheer them on during the 90-minute CNN debate, they will need to prepare for tough questions and a format that takes them out of their comfort zones, Stewart said.

Independent candidate Robert F. Kennedy Jr. didn't make the cut, so Biden, a Democrat, and Trump, a Republican, won’t have to worry about him.

Ron Klain, Biden's former chief of staff and a veteran of debate preparation, is leading Biden's sessions at Camp David, the mountain retreat in western Maryland where the president prepped for his fiery State of the Union speech in March.

A campaign spokesperson declined to comment on whether former White House counsel Bob Bauer would reprise the role of Trump he played during debate prep in 2020.

Biden's team will focus on refining the argument that Trump pursues extremist policies on abortion and other issues, is a danger to democracy, and is beholden to the rich donors writing him checks, a campaign official told Reuters.

While Biden will not shy away from attacking Trump for past actions, including his role in the U.S. Capitol riot on Jan. 6, 2021, the president wants to project himself as a wise and steady leader in contrast to Trump's division and chaos, the official said.

"What he wants to do is have that split screen, show that contrast and have President Trump be forced to account for his more extreme views," said another strategist advising the campaign, who requested anonymity to speak candidly.