Rep. Doug Collins (Ga.), the top GOP member of the House Judiciary Committee, said Sunday that Intelligence Committee Chairman Adam Schiff (D-Calif.) is the most important witness Republicans want to question in the upcoming phase of the impeachment inquiry.

"My first and foremost witness is Adam Schiff," Collins said on "Fox News Sunday," also noting that Schiff had "compared himself in the past to a special counsel" and that then-special prosecutor Ken Starr testified during the GOP-controlled House's impeachment of former President Clinton.

"[Schiff] has put himself into that position," Collins added. "If he chooses not to [testify], then I really have to question his veracity in what he's putting in his report."

"It's easy to hide behind a report," Collins said. "But it's going to be another thing to actually get up and have to answer questions."

Host Chris Wallace questioned Collins about testimony from several House Intelligence Committee witnesses who said President Trump had conditioned aid to Ukraine on investigations into former Vice President Joe Biden's son Hunter Biden and conspiracy theories about Ukrainian interference in the 2016 election.

"Before we get to the question of whether this is an impeachable offense or not, simply, do you see anything wrong with that? The president conditioning support for Ukraine - whether it was a meeting with Zelensky, whether it's military aid - conditioning support for Ukraine to that country investigating some of the president's political rivals?" Wallace asked.

"The premise of your question is based on witnesses who agree with your premise," Collins responded, saying that he himself did not agree.

Wallace continued to press Collins, asking whether aid being conditioned on an investigation of the Bidens would be an impeachable offense.

"I do not believe it, so I'm not going to answer a hypothetical which is designed to simply say that the president did something improper," Collins replied, adding that "he did nothing improper."