House Intelligence Committee Chairman Adam Schiff (D-CA) claims it would be an “abuse” of the Senate’s impeachment trial against President Donald Trump if Hunter Biden is called to testify as a witness.

“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,” Schiff, the House Democrats’ lead impeachment managers, said in an interview scheduled for a Tuesday release with CBS Evening News. “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 continued: “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.”

The lawmaker then said that Supreme Court Chief Justice John Robert, who is overseeing the proceedings in a largely ceremonial role, could rule who can be called to testify.

“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,” he said. “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.”

Hunter Biden has faced heightened scrutiny for serving as a board member of Burisma Holdings, a Ukrainian gas company at the center of allegations of corruption against him and his father. Several reports indicate Hunter was paid up to $83,000 a month by the firm despite having no expertise in the energy field.

Schiff’s comments come as the upper chamber is slated to begin their impeachment trial Tuesday afternoon.