The U.S. Capitol building in Washington, D.C., on, Dec. 12, 2019.

Under the Constitution, the impending impeachment trial of President Donald Trump must be decided by senators who are, and who have sworn to be, impartial. Because some senators, such as South Carolina Sen. Lindsey Graham, have made no secret of their refusal to be impartial, the House managers prosecuting the case against the president should move for them to be disqualified from sitting in judgment, and the chief justice, as the presiding officer, should grant that motion.

There are only three requirements in the Constitution for the conduct of impeachment trials. A two-thirds majority is needed for conviction, the chief justice of the United States presides when the president is on trial, and—most importantly—"when [the senators] are sitting for that purpose [of impeachment] they shall be on Oath or Affirmation." The oath is, of course, the requirement that the senators swear they will be impartial judges. The oath for senators, and also for the chief justice, is set forth in the Senate rules: "I solemnly swear (or affirm, as the case may be) that in all things appertaining to the trial of the impeachment of ——— ———, now pending, I will do impartial justice according to the Constitution and laws: So help me God."

Whether one considers the senators to be jurors or judges, the impartiality of decision-makers is crucial to our system of justice. The Sixth Amendment guarantees all those accused of crime "an impartial jury." As the Supreme Court has explained, "This is true, regardless of the heinousness of the crime charged, the apparent guilt of the offender or the station in life which he occupies." So, too, for judges, who Chief Justice John Roberts recently noted are charged with "maintaining the public's confidence in an impartial judiciary" and with "administer[ing] justice without respect to persons." Jurors and judges all take a solemn oath that they will render an unbiased judgment.

The Framers of the Constitution expected no less from the senators who were charged with judging cases of impeachment. In Federalist No. 66, Alexander Hamilton explained why the senators could be trusted to sit "in their judicial character as a court for the trial of impeachments." What other body, he wrote, could "preserve … the necessary impartiality between an INDIVIDUAL accused, and the REPRESENTATIVES OF THE PEOPLE, HIS ACCUSERS?"

Unfortunately, too many senators today seem ready to disregard the Constitutional requirement that they act as impartial decision-makers. Perhaps most notoriously, Graham has said, "I am trying to give a pretty clear signal I have made up my mind. I'm not trying to pretend to be a fair juror here." Meanwhile, Senate Majority Leader Mitchell McConnell affirms that his only interest is in working with the accused president to defeat the accusations. "I'm not impartial about this at all."

As is often noted, under the Constitution the Senate has the sole power of impeachment and can make up the rules. But the Senate cannot eliminate those few rules that are in the Constitution itself. The senators deciding the outcome of the impeachment trial must be "on Oath or Affirmation." They must swear that they will impartially decide the case, and they must diligently undertake to honor their oath. Of course, senators are politicians and are not immune to the public debate swirling around the impeachment inquiry. But that does not mean that they cannot render impartial justice on the evidence that is actually presented. The Supreme Court long ago recognized that, particularly in well-publicized cases, some jurors will have preconceived notions about guilt or innocence, but that does not mean that they cannot be impartial. Impartiality requires only that "the juror can lay aside his impression or opinion and render a verdict based on the evidence presented in court." It is surely not too much to ask our senators to undertake in good faith this same exercise, as the Constitution requires, and as jurors do every day.

It has sometimes been postulated that there is no mechanism to force the senators to take their oath seriously. Not so. As litigants do in selecting juries every day, the House managers can move to disqualify any senator who has prejudged this case and is unwilling to set those preconceptions aside and decide this case on the evidence. Justice Ruth Bader Ginsburg said just the other day that this standard practice should apply to the impeachment trial. "We have a process to select jurors. If a juror reveals a bias, they will be disqualified."

Who should decide the motion to disqualify any particular senator? The chief justice, of course, who the Constitution provides should preside over the trial. Much has been made of the Senate's ability to override decisions by the chief justice. In fact, the Senate rules for impeachment provide for overriding decisions by the chief justice only with respect to "questions of evidence." The motion to disqualify would rely on two provisions in the Constitution itself that the Senate cannot override: the requirement that the senators sit "on Oath or Affirmation" and the requirement that the chief justice preside. And if the senators attempt to override a ruling by the chief justice disqualifying a senator from sitting, and the chief justice acquiesces in that action, that sorry spectacle should play out in public, so that the people can judge for themselves the fairness of the proceedings.

The Framers expected that the Senate would take an impeachment trial seriously. Our citizens today deserve no less.

Gregory L. Diskant is a senior litigator at Patterson Belknap Webb & Tyler in New York and a member of the National Governing Board of Common Cause.