House Judiciary Committee Chairman Jerrold Nadler Jerrold (Jerry) Lewis NadlerTop Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence House passes bill to protect pregnant workers House Democrats postpone vote on marijuana decriminalization bill MORE (D-N.Y.), one of the House impeachment managers in President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE's Senate trial, said Sunday that Republicans looking to block or negotiate on witnesses are "part of the cover-up."

Nadler's comments came in response to a question on whether Democrats would consider calling former Vice President Joe Biden Joe BidenSenate Republicans face tough decision on replacing Ginsburg What Senate Republicans have said about election-year Supreme Court vacancies Biden says Ginsburg successor should be picked by candidate who wins on Nov. 3 MORE's son Hunter Biden to testify in exchange for calling witnesses put forward by Democrats.

Nadler dismissed the idea of negotiating on witnesses as part of the trial.

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"In any trial, all relevant witnesses must be heard," Nadler said on CBS's "Face the Nation."

.@RepJerryNadler suggests Hunter Biden should not be a witness in the Senate impeachment trial, adds that Chief Justice John Roberts should make a determination. “I’m saying that Hunter Biden has no knowledge of the accusations against the president." pic.twitter.com/Uh7w5KJAtQ — Face The Nation (@FaceTheNation) January 19, 2020

"This whole controversy about whether there should be witnesses is really a question of does the Senate want to have a fair trial, or are they part of the cover-up of the president?" he added.

The Senate will vote next week on whether to call witnesses or wait to decide on witnesses later in the process.

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"Any Republican senator who says there should be no witnesses or even that witnesses should be negotiated is part of the cover-up," Nadler said.

He added that Hunter Biden is not a necessary witness because he "has no knowledge" of the accusations against Trump.

"Their asking for Hunter Biden is just more of a smear of Hunter Biden that the president was trying to get the Ukraine to do," Nadler added.

Trump's pressure campaign in Ukraine to push for the nation to announce an investigation into the Bidens is at the heart of Democrats' impeachment allegations.

Nadler added that if Republicans want Hunter Biden to testify, Chief Justice John Roberts should make the determination.

"But no chief justice would think of admitting evidence that is not relevant. No trial judge would in any trial," Nadler said.