"Because it's very hard to say you can have [John] Bolton or you can have [Mick] Mulvaney to the Democrats when you would think that would mean the president would get one or both of the Bidens and the whistleblower," he said.
"So, I think once it gets into witness interviews, then the trial gets expansive and can take off in directions that maybe are unpredictable."
Although House Speaker Nancy Pelosi, D-Calif., has yet to tell her colleagues who she plans to name as impeachment managers, she suggested that the House will vote Wednesday on a resolution to name the managers and transmit the articles to the Senate.
McCarthy told Smith that -- for the sake of analogy -- in every criminal proceeding, no case ever starts off with a guarantee there will be witnesses.
"There is a whole stage of motion practice and the prosecution has to defend the case," said McCarthy. "The defense gets a chance to argue that they haven't met the standard -- that, even if everything they allege is true, it doesn't meet the standard of whatever it is you have to prove.
"I think that's what the president's position will be here."
McCarthy told Smith that Republicans would like to skip witnesses altogether because they "really can't be scripted."
"So, if this gets extended in time and space and there are developments outside the parameters of the Senate trial, they could end up affecting the Senate trial in unpredictable ways. I think that is why people would like to get it done as quickly as they can get it done," he observed.
However, McCarthy noted that if Democrats had a truly impeachable offense or were alleging a crime, "wild horses couldn't have stopped Nancy Pelosi from getting these articles of impeachment to the Senate as quickly as she could have because they would have wanted a trial."