There are four witnesses who could help fill those gaps: the president’s former national security adviser, his acting chief of staff, the senior adviser to his acting chief of staff and a top national security official in the Office of Management and Budget (OMB). Each has direct knowledge regarding the charges against the president and should testify under oath at a Senate trial.

Let me be clear: I do not know what their answers would be, but I want to hear from them, and so should every senator and every American. We cannot allow the full truth to evade this trial only to be revealed in some future memoir or committee hearing.

Foremost among these four is former national security adviser John Bolton. Public testimony under oath revealed that Bolton abruptly ended a meeting with Ukrainian officials concerning withholding congressionally approved military assistance, as well as a White House visit, later characterizing the discussions as a “drug deal” he wanted no part of. He was alarmed enough to order that the top National Security Council lawyer be informed of what acting chief of staff Mick Mulvaney and Gordon Sondland, the U.S. ambassador to the European Union, were doing.