"He can't fake his way through 90 minutes," Chen, a research fellow Hoover Institution and a CNN contributor, told CNN's Party People podcast hosts Kevin Madden and Mary Katherine Ham.
Trump participated in debates throughout the Republican presidential primary schedule, emerging largely unscathed in the polls after fighting off more than a dozen GOP presidential primary opponents, and locking up the party's nomination by May.
Chen, a Republican policy wonk who worked for former Massachusetts Gov. Mitt Romney and Florida Sen. Marco Rubio on their respective presidential campaigns, said the general election debates present a different challenge -- in part because he'll have just one other opponent and an increased focus on substance.
"The 90-minute general election format makes it a lot harder to one line your way through it," Chen said, adding, "I think it's going to be very, very hard for Donald Trump to spend 90 minutes bumper-stickering. I think he's got to have something."
Chen said Clinton faces her own challenges in the general election debates -- the first of which is on Monday at Hofstra University in Hempstead, New York.
"I think for Hillary Clinton the challenge is: How does she expose the deficiencies in what he said or proposed without sounding pedantic?" Chen said. "She's going to sound like the teacher that nobody likes. And nobody wants to hear the teacher they don't like lecturing them for 90 minutes."
Chen, whose background is in developing conservative policy ideas, said that if Trump wins the presidency then the real estate mogul would likely outsource setting his policy agenda to his running mate Mike Pence or House Speaker Paul Ryan.
"Either way, Paul Ryan plays a big role," Chen said.
"Paul Ryan is like 'How many thankless jobs can one man take?'" Ham added.
To hear what Chen predicts a Republican Congress would do under a would-be President Clinton, how he got involved with politics to begin with and what policy he thinks both sides of the aisle can agree on, listen to CNN's Party People.
Party People is a new podcast from CNN where a pair of conservative CNN contributors talk to influential voices about the future of conservative politics and the Republican party.
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