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.

Millions of eyeballs will be on Thursday's debate -- here's what to watch for as Biden and Trump face off.

Will any gaffes or knockout punches break through? Conversations on policy are unlikely to break through, but viewers could remember a serious lapse or a zinger that lands.

Will character or policy rule the day? Both sides have lobbed personal attacks -- Biden has dubbed Trump a "convicted felon" and threat to democracy, and Trump has essentially called the president, without evidence, the leader of a crime syndicate. Will the debate stay focused on issues like abortion, immigration and the economy? Or will the two candidates opt for a more scorched-Earth stance?

Typically, a sitting president is debating a governor or senator, with the incumbent's record the main focus. But Trump also spent four years in the White House. Whose record will get more scrutiny?

The debate is happening uniquely early in an election cycle. It remains unclear if any big moments will be washed away by November, or if it'll help set the tone heading into the summer.

The debate is also featuring a novel format -- microphones will be cut off when candidates aren't recognized to speak, and there will be no audience to feed off of. That could deprive Trump of the energy he seeks, helping Biden. But the microphone cutoffs could help "enforce some discipline" on Trump, said GOP strategist David Kochel.

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