Sen. Sherrod Brown, D-Ohio, said Sunday that he's "fine" with Republicans' calling former Vice President Joe Biden's son, Hunter Biden, as a witness in President Donald Trump's impeachment trial.

"We take the position that we want to hear from witnesses," Brown said on CNN's "State of the Union." "I don't know what Hunter Biden has to do with the phone call the president made."

He added: "I think many Republicans think that's a distraction. That's what Republican senators tell me quietly."

Democrats and Republicans have been battling for weeks over just how much more information will be presented at the trial, which is set to begin Tuesday.

Brown said it would be unjustifiable not to call additional witnesses or to introduce any new information related to the trial. He echoed Senate Minority Leader Chuck Schumer, D-N.Y., in calling for testimony from "people who were in the room" and were "eyewitnesses" to the president's efforts regarding Ukraine.

Schumer has said he wants to hear from four current and former Trump administration officials, including acting White House chief of staff Mick Mulvaney and former national security adviser John Bolton. Bolton has said he would testify if subpoenaed by the Senate. Trump has suggested that he would block Bolton's testimony.

Senate Majority Leader Mitch McConnell, R-Ky., has said the question of additional witnesses and documents will be determined by a vote after initial arguments are presented by both sides. The House passed two articles of impeachment against the president: one alleging abuse of power for pressuring Ukraine to investigate the Bidens and the other alleging obstruction of Congress.

McConnell has suggested that Hunter Biden could be called as a witness — something Trump and his allies have suggested they want to have happen.

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Speaking on Fox News' "Sunday Morning Futures," Sen. Ted Cruz, R-Texas, chairman of the Judiciary subcommittee on the Constitution, called Hunter Biden "such an important witness," although he did not commit to voting to call witnesses. Sen. Lindsey Graham, R-S.C., said on "Fox News Sunday" that Republicans "can look at" the Bidens "outside of impeachment."

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Sen. Dick Durbin, D-Ill., a member of the Judiciary Committee, said on NBC's "Meet the Press" that the issue of whether to call the younger Biden is in dispute among Republicans.

"We've been told even within their caucus there's dispute as to whether that's really in their best interest," he said.

Trump and allies, led by his personal attorney Rudy Giuliani, have argued for months that Hunter Biden's service on the board of the Ukrainian gas company Burisma, at a time when his father oversaw Ukraine policy in the Obama administration, was corrupt. Most notably, they have advanced the unsubstantiated claim that as vice president, Joe Biden, now a leading presidential contender, took steps to protect his son from an investigation of the company.

But Bloomberg News, citing documents and a former Ukrainian official, reported that the Burisma investigation had been dormant for more than a year before Biden called for Viktor Shokin, Ukraine's top prosecutor at the time, to step down. PolitiFact, meanwhile, reported that it found no evidence to "support the idea that Joe Biden advocated with his son's interests in mind."

Biden's 2016 call for Shokin's ouster was widely backed by the international community, as Shokin was seen as ineffective and was accused of refusing to investigate corruption. He was later removed by parliament.

It was during a phone call on July 25 that Trump asked Ukrainian President Volodymyr Zelenskiy to investigate the Bidens. The call set off a series of events that led to Trump's impeachment.

Speaking on ABC's "This Week," Sen. Cory Booker, D-N.J., a member of the Judiciary Committee, said, "These assaults on the Biden family are not relevant to what's at issue in this case."

He added: "And this is an invitation to try to muddle waters."

House Judiciary Committee Chairman Jerry Nadler, D-N.Y., said on CBS' "Face the Nation" that the younger Biden "has no knowledge of the accusations against the president."

"Did the president, as we said — as the evidence shows that he did — betray his country by conspiring with a foreign country to try to rig the election?" asked Nadler, a House impeachment manager. "Hunter Biden has nothing to say about that. Their asking for Hunter Biden is just more of a smear of Hunter Biden that the president's trying to get the Ukraine to do."

The battle over witnesses and documents intensified last week after the House released a trove of documents from Lev Parnas, an indicted associate of Giuliani's, who spoke extensively with MSNBC and CNN about his efforts to dig up dirt in Ukraine. Then, on Thursday, the nonpartisan Government Accountability Office said in a decision that the Trump administration broke the law by withholding nearly $400 million in aid to Ukraine.

Republicans pushed back on the idea of Parnas' testifying. Sen. David Perdue, R-Ga., said on "Meet the Press" that Parnas' information was "secondhand" and "a distraction," even though Parnas had worked closely with Giuliani in Ukraine.

Sen. John Cornyn, R-Texas, a member of the Judiciary Committee, said on "Face the Nation" that he "would be careful before crediting the veracity of somebody who is under indictment in New York, the Southern District of New York, and who's trying to get leniency from the prosecutor and who has ties to Russian oligarchs."

Cornyn added: "There's no question that there have been a series of grifters and other hangers-on that have associated themselves with the president's campaign or claimed to have special relationships with the president."

Asked whether he had any questions for Giuliani about Parnas, Cornyn said, "Not relative to the impeachment."

"That's a relationship that causes many of us to sort of scratch our heads," he said. "But I'd say he's not relevant to the articles and what the Senate's going to be asked to do: impeaching a president for the third time in American history for a non-crime over events that never occurred."