Biden stepped off Air Force One in Georgia about 3:15 p.m. to greet a group of supporters on the tarmac applauding his arrival. The president was donning his signature aviators and a navy-blue suit.

He spent several moments shaking hands with Atlanta Mayor Andre Dickens, former mayor Keisha Lance Bottoms, Stacey Abrams and Jason Carter, Jimmy Carter's grandson.

As he made his way to CNN's studio, Biden made an unexpected stop to shake hands with a group of cheering supporters. The group held "Dark Brandon" cardboard cutouts and chanted "Let's go, Joe!" and "Four More Years!"

Biden moved down the line to shake hands and meet people for nearly 10 minutes.

-ABC News' Molly Nagle

As Biden and Trump prepare to square off in their first political debate in four years, historians and experts contend the matchup may have a small but crucial impact on the election.

Aaron Kall, director of debate for the University of Michigan's Debate Program, told ABC News the majority of those who tune in are likely already locked into a preferred candidate.

"Nothing that occurs during the 90-minute debate is going to change or influence who they're going to vote for," he said.
However, Kall and other experts ABC News spoke with said there is still a smaller group of undecided voters who do tune in and can be swayed by the performances.

With the last two presidential elections decided by just tens of thousands of votes in a few states -- many cast by independent voters -- candidates' debate strategies have become laser-focused on courting that group, according to Julien Labarre, administrator of the University of California Santa Barbara's Center of Information Technology & Society.

"What we see is people who were not thinking of going to vote being turned into voters," he told ABC News. "Spurring people into participation, we do see that kind of effect."

-ABC News' Ivan Pereira

After tonight's debate, there will be a rush to anoint a "winner" and a "loser," but the only way we can really do that is once we have data on how the debate will actually affect people’s votes. To that end, 538 partnered with Ipsos to poll the same group of likely voters both before and after the debate to see how their attitudes change. Here are some of the key findings from our pre-debate poll, which was conducted using Ipsos' KnowledgePanel.

First, we asked respondents to rate how well they thought each candidate would perform in the debate tonight on a five-point scale. On average, Trump got a score of 2.96 out of 5, and Biden got a score of 2.58 out of 5. In other words, expectations are significantly lower for Biden tonight, which could end up helping him -- even a so-so performance from Biden would exceed most people's expectations.

It looks like the reason people have such low expectations for Biden is his advanced age. We also asked respondents to grade each candidate's physical, mental and emotional fitness on a five-point scale. On average, Biden got just a 2.3 out of 5 on physical fitness and a 2.4 out of 5 on mental fitness. Trump bested him on both of those measures, but Trump got only a 2.6 out of 5 on emotional fitness, which was lower than Biden's score.

We also asked voters what issues would have the most impact on their vote. Fifty percent ranked inflation or increasing costs as one of their top three issues, while 37% included immigration. Voters also said Trump would do a better job handling those issues than Biden, so it will be especially important for the president to show strength on these issues tonight.

Finally, we asked voters which candidates they were considering supporting. Heading into the debate, 44.8% of voters are at least considering voting for Trump, 44.5% are at least considering voting for Biden and 18.5% are at least considering voting for independent candidate Robert F. Kennedy Jr., who failed to qualify for the debate. (Respondents could say they were considering multiple candidates, which is why these numbers add up to more than 100%.) We'll ask voters the same question after the debate to see whether these numbers shift.

-538's Nathaniel Rakich

The debate between Biden and Trump marks one of the few foreseeable opportunities to change a race characterized by stagnant polls.

Literal history is in the rearview in the race, including 34 felony convictions for Trump in New York -- that leaves just the debates, the party conventions and Trump's sentencing as the only dates on the calendar that the campaigns could circle as opportunities to try to gain an edge.

"If you're looking at the calendar for the next five months, this is one of those moments. And somebody's going to take advantage of it," Chip Saltsman, a GOP strategist who worked on former Vice President Mike Pence's now-suspended presidential bid, told ABC News.

Read more here.

Biden had a poor start to this debate, and if he ends up being viewed as the "loser" tonight, he'd be just the latest incumbent president to go through this. Incumbent presidents like Ronald Reagan in 1984, George W. Bush in 2004, Barack Obama in 2012 and Trump in 2020 all lost 2 to 3 points in the polls after their first debate.

Reagan, the last president to face real questions about his age -- he was 73 in 1984 -- had a meandering performance that prompted questions about whether he had gotten too old for the job. Reagan shot back in the second debate against Democratic nominee Walter Mondale, who was in his mid-50s, with the famous line: "I will not make age an issue of this campaign. I am not going to exploit, for political purposes, my opponent's youth and inexperience." Reagan, however, also had a roaring economy, which helped him win a landslide reelection victory.

-Geoffrey Skelley, 538