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."

How can Mr. McConnell make such a claim without having heard from Mr. Bolton? Remember that the diplomat Fiona Hill testified at the House impeachment hearings that Mr. Bolton called the pressuring of Ukraine by the administration a "drug deal" and said he wanted no part of it. Mr. Bolton himself has said that he possesses new information that has not been revealed. He even gave a speech saying that some of Mr. Trump's foreign policy decisions were made in his self-interest, not in the interest of the American people. Particularly after the United States' killing of Maj. Gen. Qassim Suleimani of Iran, such questions have arisen once again.

And how can Mr. McConnell make such a claim without having heard from the most important witness of all, Mr. Trump? The president has been too scared to testify, and too scared to let anyone else in his administration testify. This is not a particularly compelling demonstration of innocence. When the House was holding impeachment hearings, Mr. Trump said he wanted the acting chief of staff, Mick Mulvaney, and Energy Secretary Rick Perry to testify. Even that pretense is now gone.

There is only one possible explanation for this behavior: He is afraid of the truth. Otherwise, what argument can there be for refusing to hear from a central witness like Mr. Bolton, who other witnesses have indicated was exceptionally concerned about the suspension of military aid to Ukraine?

Making matters worse, Mr. McConnell is a lawyer (as are nearly 50 of his Senate colleagues). Yet clearly being a lawyer does not give one a monopoly on truth seeking, as his Republican colleagues with law degrees are proving.Lindsey Graham, for example, said if there were evidence of a quid pro quo, he'd want to investigate, only to turn around after such evidence emerged to try to stop that very inquiry.

Marco Rubio deserves particular opprobrium. He has said Mr. Bolton cannot testify, reasoning that the Senate "inquiry should be based on the testimony" heard by the House. "We are acting on articles of impeachment," he declared. "We should be constrained by the information that those articles are based on." His ostrichlike behavior would appall the founders, who believed, in the words of Federalist 65, that such "strict rules" could never govern impeachments.

Mr. Rubio would have an even harder time explaining the 41 witnesses who testified in Andrew Johnson's Senate impeachment trial, or the three who testified in Bill Clinton's. Neither of those impeachments involved a president who had ordered those very witnesses not to cooperate with the House. It's quite rich to say the witnesses should have testified in the House when the defendant in the proceedings blocked them from doing so.

There's one bright spot: another senator who happens to be a lawyer, too. Mitt Romney recently said that the Senate should hear from Mr. Bolton. A handful of other Republican senators could join him and ensure a real trial, with witnesses and documents alike. Lisa Murkowski, Cory Gardner and Rob Portman — all of whom happen to be lawyers — would be a good start.The two of us are lawyers and became friends and writing partners out of our shared reverence for the rule of law. We have very different politics, but we believe our commitment to this principle far eclipses the rest. The Constitution imposes upon the Senate a duty to "try all impeachments," and so a real trial — with all relevant testimony and evidence — is what is required.

This week, Mr. Bolton, himself a lawyer, and recognizing the nature of the Senate's crucial constitutional obligation, has taken a critical step in the right direction. It's our hope that Americans will recognize that our commitment to the rule of law is what holds us together.

The truth may not set the president free, but the Constitution is meant to keep the country free, and a fair and impartial trial is what must take place here.

Neal Katyal, former acting solicitor general and a law professor at Georgetown, is the author of the book "Impeach: The Case Against Donald Trump." George T. Conway III is a lawyer and an adviser to the Lincoln Project. This appeared in The New York Times on January 7, 2020, following Mitch McConnell’ s assertion that he had the votes to begin President Trump's trial without committing to calling witnesses or admitting new evidence.

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January 7, 2020

Voices4America Post Script. This is an Emergency. Demand a #FairTrial, that #JohnBolton be subpoenaed, and that the Senate move forward with #Witnesses.Call the Capitol Switchboard at (202) 224-3121. An operator will connect you directly with the Senator you choose. #StopMcConnell