What's Worrying Hillary Clinton's and Donald Trump's Campaigns Before the First Debate

ORLANDO, Florida -- While Hillary Clinton and Donald Trump prepare for their first debate on Monday, aides in both their campaigns are expressing concern over how it will all go down.

The Trump campaign’s major concern: the candidate’s limited preparation. The Clinton camp’s biggest worry: that the Republican presidential nominee will be asked softer questions than Clinton.

“My biggest concern continues to be a low bar set for him on expectations,” Clinton’s communications director Jennifer Palmieri told reporters during a flight to Orlando, Florida, today, referring to the debate at Hofstra University in Hempstead, New York, on Sept. 26 — the first of three face-offs between the two White House hopefuls.

“People accommodate the questions and lower the bar on their questions to suit the candidate in front of them, and that’s what’s happened with Trump in the past. And they ask Hillary Clinton a set of much harder questions,” Palmieri said. “They ask him a set of easier questions because he has not put forward detailed ... material you can parry him on.”

Her concern stems from what the campaign considers to have been unfair treatment of Clinton during a recent NBC News national security forum that featured both candidates fielding questions from a moderator and audience members in separate segments.

Clinton has been preparing for the debate for weeks, and she has no public campaign events planned for the rest of the week so she can continue studying. Aides say the former secretary of state — who is known for doing her homework — has been poring over briefing books and practicing in mock debates with a Trump stand-in.

She has been studying footage of Trump’s past debates. From this, Palmieri said, they believe the Republican nominee tends to do better in multicandidate settings, where he can hang back and pick his moments.

The campaign is also concerned about Trump’s unpredictable nature.

“You’re not sure who is going to show up. He may be aggressive or laid back,” Palmieri said.

And while it is common for campaigns to lower expectations before debates and other high-profile events, sources told ABC News that Trump’s preparation for the 90-minute one-on-one battle has been slim — so much so that his senior staff has become concerned about his lack of focus.

While Trump has boasted that he will win Monday’s debate, senior sources said he has not delved into policy or participated in mock debates. During his Sunday meetings in Bedminster, New Jersey — where he huddles with campaign chief Steve Bannon, campaign manager Kellyanne Conway, Roger Ailes and other advisers, including members of his family — the prep has become more of a discussion of topics and themes and not specific in nature. (Those sessions often become storytelling sessions rather than actual, intense preparation, according to senior sources.)

Trump, sources said, was given an iPad loaded with footage of old Clinton debates.

Unlike Clinton, who is spending almost the entire rest of the week preparing for Monday, Trump has a busy campaign schedule this week, with no large blocks of time carved out for debate prep. Aides said his debate sessions planned for this weekend have been moved to Trump Tower in New York City, away from his Bedminster estate and golf course, to help him focus.

The Trump campaign recently added a Saturday evening event in Roanoke, Virginia.

ABC News’ Tom Llamas and John Santucci contributed to this report.