Intelligence Committee Chairman Adam Schiff, one of the House impeachment managers, said the question of whether witnesses are heard in President Trump's Senate impeachment trial may have to be settled by Supreme Court Justice John Roberts, who will be presiding over the trial. But, he warned that witnesses like Hunter Biden, former Vice President Joe Biden's son, are "immaterial" and would constitute an "abuse" of the Senate trial.

In an exclusive interview, "CBS Evening News" anchor Norah O'Donnell asked Schiff whether Democrats are prepared for who Republicans might want to call as witnesses. The Democrats have requested four witnesses: former national security adviser John Bolton; acting White House chief of staff Mick Mulvaney; senior adviser to Mulvaney, Robert Blair; and Office of Management and Budget official Michael Duffey.

One of the names that's been floated for Republicans is Hunter Biden, who was on the board of Ukrainian energy company Burisma while his father was vice president.

"It would certainly be fair for the president and his team to be able to call witnesses that can provide material information on the charges. It would not be appropriate for the president to seek to call witnesses merely to try to perpetuate the same smear campaign that was foiled when his plot was discovered," Schiff told O'Donnell. "Hunter Biden, for example, can't tell us anything about whether the president withheld military aid, whether he withheld that aid to coerce Ukraine to conduct political investigations. Or why he wouldn't meet with the president of Ukraine."

Schiff added, "The only purpose in putting him on their list is they wish to trade material witnesses, like Mr. Bolton and Mulvaney and others for immaterial ones that will allow them to continue to attack a political opponent. That's an illegitimate abuse of the trial. And the chief justice, who may have an opportunity to rule on materiality of witnesses, as well as the senators, should not permit that kind of abuse."

Watch more of O'Donnell's interview with Schiff and three other impeachment managers, Representatives Val Demings, Zoe Lofgren and Jason Crow, on the "CBS Evening News" at 6:30 p.m. ET and on CBSN at 10 p.m. ET Tuesday.