Democratic presidential candidate Hillary Clinton waves as she boards her campaign plane at Westchester County Airport, in White Plains, N.Y., on Wednesday, Sept. 21, 2016. Matt Rourke AP
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Paul Begala, chief strategist to Bill Clinton who is an adviser to a pro-Hillary Clinton super political action committee, said Clinton relied on doing her homework and the facts about the issues, which made an opponent like Trump frustrating to debate.

“What makes her comfortable is the facts,” Begala said. “It’s as if God or the devil or fate constructed the perfect opponent to drive her crazy.”

Trump has downplayed the importance of debate preparation and has several events scheduled this week, including a Saturday night campaign rally in the swing state of Virginia.

Clinton aides acknowledge that Trump did well in Republican primary debates when he was on the stage with several opponents but they don’t know how that will translate to a one-on-one exchange.

“Donald Trump, certainly during the Republican primary debates, showed his ability to win debates purely based on his showmanship and his flair for the dramatic,” Fallon said.

Clinton aides say they learned something about how Trump debates from watching the recent Commander-in-Chief Forum, the candidates’ first joint appearance.

“They weren't together but they were one-on-one in the same hour . . . we thought that was useful,” Palmieri said. “What we have never seen is one-on-one.”