Trump's legal team has warned repeatedly during the two-day question-and-answer session that allowing new witnesses would be a legally fraught, time-consuming effort that would not change the outcome of the trial where 67 votes are needed to convict Trump.

"It's not a question of a lot of people talking right now about John Bolton," he said. "The president would have the opportunity to call his witness, just as a matter of fundamental fairness."

The Senate will vote on Friday on whether or not to allow new witnesses and documents as part of the trial. Democrats will need to win over four Republican senators in order to call witnesses.

Under the rules for the trial passed last week, any witnesses who senators agree to subpoena would first be deposed behind closed doors. The Senate would then decide if the individual needed to testify. But the rules do not put a cap on how long that process can take.