The White House Correspondents' Association is "deeply concerned" that CNN has barred travel pool reporters from being inside the studio and close to Biden during all of tonight’s historic presidential debate.

Despite "repeated requests" for White House pool access, CNN will allow only one print reporter to enter during a commercial break for a brief observation.

"That is not sufficient in our view and diminishes a core principle of presidential coverage," president of the WHCA Kelly O’Donnell said in a statement Thursday.

"The pool is there for the 'what ifs?' in a world where the unexpected does happen,” O’Donnell said.

-ABC News' Emily Chang

Just because their microphones will be turned on only when it's their turn to speak, it doesn't mean there won't be interruptions, and campaign staffers of both candidates are preparing for it.

Since the candidates are still only 8 feet apart, there is a scenario where the viewer may not hear an interruption, but the other candidate and the moderators will.

Advisers from both campaigns told reporters that's something they've acknowledged or discussed in debate preparations.

The Biden campaign said it is hoping the president just flat-out ignores any distractions or comments that Trump makes.

The Trump campaign said it is aware that tonight they could play this both ways: The former president could stay on message when his microphone is turned on but make off-hand comments to irk Biden when his microphone is turned off.

When it comes to the traditional handshake between the candidates before the event, both sides are being coy.

One Biden adviser said, "I wouldn't do it," while an adviser for the Trump campaign responded simply by laughing.

-ABC News' Rachel Scott

Biden and Trump are navigating different expectations heading into the debate -- though Republicans have largely set the standards for each.

Polls show that voters share concerns about Biden's age (81 years old) and fitness for office, and Republicans have for years cast the president as a dithering man. Showing vitality, as he did during this year's State of the Union, and nimbly mixing it up with Trump, could help alleviate those worries, Democrats told ABC News.

Trump, meanwhile, has been working overtime to set his own expectations. He's repeatedly demeaned CNN -- the host -- to suggest he'll be debating behind enemy lines. And he's emphasized his unfounded claims that Biden will be on some kind of drug to enhance his performance, seemingly to undercut the prospect of a good performance by the president. Some Republicans have also been highlighting Biden's extensive resume of running races and debating.

Still, Trump's allies are setting high expectations for him, with senior adviser Jason Miller telling ABC News that Trump has demonstrated "elite stamina."

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Trump arrived in Atlanta just before 5:30 p.m. As he exited the plane, he raised his fist and clapped his hands.

He went straight into his motorcade without approaching or greeting nearly 200 supporters who gathered to welcome him.

Accompanying him were his advisers Susie Wiles, Steven Cheung, James Blair, Jason Miller, Chris LaCivita, and Corey Lewandowski. The only lawmaker on the plane with him was Republican Rep. Matt Gaetz of Florida.

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