Even Richard Nixon allowed the key figures behind the Watergate scandal to speak to Congress, and he eventually turned over incriminating portions of his Oval Office recordings to investigators. But Mr. Trump has stonewalled Congress and inhibited our ability to seek justice by demanding that those closest to the center of the Ukraine scandal stay silent.

Senators must be allowed to subpoena relevant witnesses and submit questions to them directly. The Senate should not vote on any article of impeachment or consider a motion to dismiss the trial until we have reviewed the additional testimony and evidence that Mr. Schumer has requested.

I have never been in a courtroom where the accused can unilaterally block witnesses from testifying or prohibit prosecutors from asking witnesses questions. No court would allow a trial to proceed this way, and neither should any member of the Senate.