Senate Democrat says 'fine' if Hunter Biden were to testify in impeachment trial

Brown, though, added in his comments on "State of the Union" that he's not sure what value testimony from the son of former Vice President Joe Biden would provide.

"I think you bring in -- We take the position that we want to hear from the witnesses. I don't know what Hunter Biden has to do with the phone call the President made.." the Ohio Democrat told CNN's Brianna Keilar, adding: "The point is we need witnesses, we need to know who they are with the right to call witnesses, additional witnesses later. But I don't understand how you come to the American public, make the case that this is a real trial, if there are no witnesses and there is no new evidence."

The impeachment trial is set to begin in earnest Tuesday and the Senate is expected to pass a resolution outlining the rules of the impeachment trial with only with Republican votes. Though the text of the resolution hasn't been released, it's expected to punt the question of calling witnesses until after opening arguments and senators' questions, while including an opportunity to vote on whether the Senate should have witnesses.

Trump and congressional Republicans have criticized House Democrats for not allowing Hunter Biden to be called as a witness during the impeachment hearings.

Democrats have called for testimony from four witnesses, including former national security adviser John Bolton, acting White House chief of staff Mick Mulvaney, senior adviser to the acting White House chief of staff Robert Blair and Office of Management and Budget official Michael Duffey. They have also pointed to recently released documents provided to the House by indicted Rudy Giuliani associate Lev Parnas as evidence that the trial should include witnesses and additional documents.

Trump faces House-approved charges of abuse of power and obstruction of Congress.

Democrats say Trump abused his office by directing a pressure campaign for Ukraine to announce an investigation into Joe Biden in exchange for $400 million in US security aid and a White House meeting. Trump, Democrats say, sought to bolster his reelection efforts through the misconduct and then stonewalled congressional investigators to coverup it up.

There is no evidence of wrongdoing by either Biden in Ukraine. The President has denied doing anything improper.