The importance of John Bolton’s offer to testify if subpoenaed in the impeachment proceedings against President Trump cannot be overstated. In a single stroke, Mr. Bolton, the former national security adviser, elevated truth and transparency over political gamesmanship.

The Senate must take him up on his offer, as well as demand the testimony of President Trump and the administration officials he has barred from testifying. The Senate majority leader, Mitch McConnell, reportedly has the votes to proceed with the trial despite no agreement with Democrats on new witnesses and to leave it a question to take up after opening arguments. The Senate still must declare that it will call witnesses during the trial.

Everyone — Republicans, Democrats and independents — must know that these crucial witnesses will be heard.

The core principle behind the rule of law is that justice is blind and partisan identity should not influence a trial’s outcome. But anyone watching Mr. McConnell twist himself into knots in trying to block witnesses and documents has to wonder whether this notion ever took root in his mind. He has gone so far as to say that “there will be no difference between the president’s position and our position as to how to handle this to the extent that we can.” He also said, “There’s no chance the president is going to be removed from office.”