Rep. Adam Schiff, the lead House manager in President Trump’s impeachment trial, said Sunday that the main strategy of the president’s defense team is to argue against calling witnesses to testify before the Senate and to “deprive the public of a fair trial.”

“I think they're definitely afraid of what witnesses will have to say, and so their whole strategy has been deprive the public of a fair trial,” Schiff, a California Democrat, said on ABC’s “This Week.” “They don't frame it that way, but that's in essence, it. They have a very heavy burden, though, with that, because the American people understand what a fair trial is. A fair trial requires witnesses.”

The argument between the House managers and Trump's defense team over calling witnesses has been a key battle in the impeachment trial.

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Democrats want a number of key current and former Trump administration members to come before the Senate, including former National Security Advisor John Bolton, acting White House Chief of Staff Mick Mulvaney, and other key administration figures. Bolton has already voiced his willingness to testify.

While it seems unlikely that the House managers will get enough Republican support to get the witnesses to testify, Republicans have said that they would want to call Hunter Biden to testify. Biden, the son of former Vice President Joe Biden, was a member of the board of a Ukrainian natural gas company that Trump asked leaders in Kiev to investigate.

Schiff on Sunday said he has no issue with Republicans calling witnesses as long as they are not “irrelevant.”

“I think the president has the right to call relevant witnesses, just as we do in his defense,” Schiff said. “He doesn't have the right to call irrelevant witnesses or witnesses who aren't fact witnesses.”

In a rare Saturday session of Congress, Trump's legal team rebutted allegations that the president abused his power when he asked Ukraine to investigate former Vice President Joe Biden, a potential political rival, and then obstructed Congress as it tried to investigate. The lawyers are mounting a wide-ranging, aggressive defense asserting an expansive view of presidential powers and portraying Trump as besieged by political opponents determined to ensure he won't be reelected this November.

“They're asking you to tear up all the ballots across this country on your own initiative, take that decision away from the American people," White House counsel Pat Cipollone said.

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Republicans accused Democrats of cherrypicking evidence and omitting information favorable to the president, casting in a nefarious light actions that Trump was legitimately empowered to take. They focused particular scorn on Schiff, trying to undercut his credibility.

Schiff has so far brushed off the criticism from Trump and other Republicans, arguing that it takes “moral courage” to stand up to the White House.

“I made the argument that it's going to require moral courage to stand up to this president,” he said.

The Associated Press contributed to this report.