As President Donald Trump's impeachment trial opened, Chief Justice John Roberts swore in all the members of the U.S. Senate who made an oath to "do impartial justice according to the Constitution and laws, so help me God.” The oath is not an empty ritual; it's a Constitutional requirement whose integrity must be enforced.

If a senator raises his or her right hand and mouths the words, but really means, “This is a political process and I don’t take my promise to be impartial seriously,” that's false swearing. We can't pretend it doesn't matter. Such cynicism degrades the process and the rule of law, which rest upon the integrity of words — oaths, contracts, treaties, and the Constitution.

Before taking the oath, several senators made prejudicial statements indicating they won't abide by it, don't want to hear any more evidence, and are anything but impartial.

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“Everything I do during this [process], I'm coordinating with White House counsel,” said Sen. Mitch McConnell Addison (Mitch) Mitchell McConnellObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE (R-Ky.). “There is no chance the president is going to be removed from office," he added, openly admitting “I’m not impartial about this at all.”

“I have made up my mind,” said Sen. Lindsey Graham Lindsey Olin GrahamSenate Republicans face tough decision on replacing Ginsburg Democratic senator calls for eliminating filibuster, expanding Supreme Court if GOP fills vacancy What Senate Republicans have said about election-year Supreme Court vacancies MORE (R-S.C.), “I’m not trying to pretend to be a fair juror here,” he added, promising to make impeachment “die quickly.” Other senators in both parties have declared which side they’re on, and how they'll vote.

Admitting to partisan bias is one thing: It's honest.

Seeking a predetermined verdict and precluding admission of material witnesses and evidence is something else: It violates the oath.

Legally, swearing an oath falsely is perjury, for which President Bill Clinton William (Bill) Jefferson ClintonBarr says Ginsburg 'leaves a towering legacy' Trump reacts to Ginsburg's death: 'An amazing woman who led an amazing life' Jimmy Carter remembers Ruth Bader Ginsburg as 'a beacon of justice' MORE was impeached. The gravity of impeachment demands that senator-jurors hear and adjudicate all evidence of guilt or innocence without partiality. It's time to stand up and defend that principle.

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As Harvard Law professor Lawrence Lessig has pointed out, any senator could have objected to another senator taking the oath at the swearing in. That would have required Chief Justice Roberts to rule on the challenge. Since no objections were raised, he wasn't required to take judicial notice of senators’ prior statements. He just presumed their good faith and took them at their word.

There was mild push-back. Public Citizen filed a complaint with the Senate Ethics Committee, requesting an investigation into whether McConnell should recuse himself. Rep. Jackie Speier Karen (Jackie) Lorraine Jacqueline SpeierOvernight Defense: House to vote on military justice bill spurred by Vanessa Guillén death | Biden courts veterans after Trump's military controversies House to vote on 'I Am Vanessa Guillén' bill Overnight Defense: Trump's battle with Pentagon poses risks in November | Lawmakers launch Fort Hood probe | Military members can't opt out of tax deferral MORE (D-Calif.) called on McConnell to do so voluntarily. But there's zero prospect of voluntary recusal. We need an enforceable mechanism.

Fortunately, one exists.

In jury trials, it's permitted to challenge and remove a juror even after the swearing in. Similarly, any senator, or any of the House managers, can challenge a senator-juror and make a motion to remove him or her for cause, even after the oath is taken.

For 20 years, beginning in 1864, senators swore an oath to support the Union. They could — and did — challenge one another on grounds of integrity and competence to take the oath. They took that duty seriously. “A false oath, taken with our knowledge, would compromise the Senate," said Sen. Charles Sumner. "We who consent will become parties to the falsehood.”

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Senators and House managers in President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE’s impeachment trial have a similar duty now. If they consent to false oaths, it will compromise the Senate and the process. They can challenge senator-jurors who flout the impartiality oath simply by making a motion to bar them from serving as jurors. The Chief Justice would then have to rule on those cases.

It’s not too late for senators to use this power, and it’s imperative that they do.

It wouldn't necessarily be partisan or biased against the president. In the 1868 impeachment trial of Andrew Johnson, the oath and the power to challenge oath takers facilitated for acquittal by reducing partisan pressures on the process.

If a senator or a House manager moved to challenge a senator-juror, how could Chief Justice Roberts possibly fail to take it seriously? Just as excluding relevant testimony or documents undermines a tribunal’s legitimacy, so does suborning perjury and seating jurors who prevaricate, evade and dissemble as they swear impartiality.

Supreme Court Justice Benjamin Cardozo wrote in 1933 that if someone gets on a jury by saying things that are "willfully evasive or knowingly untrue, the talesman, when accepted, is a juror in name only... His relation to the court and to the parties is tainted in its origin; it is a mere pretense and sham.”

Those same charges have been leveled at this impeachment process.

To protect its integrity and the Senate's, the first line of defense is to hold oath takers accountable for what they swore. It only requires the courage of a senator or a House manager to make a motion.

Jonathan Granoff is president of the Global Security Institute, and representative of the World Summits of Nobel Peace Laureates to the United Nations. He chairs the Task Force on Nuclear Nonproliferation of the International Law Section of the American Bar Association, and he is a fellow of the World Academy of Arts and Science. He has testified as an expert before the U.S. Congress, the United Nations, Canadian Parliament and U.K. Parliament. He was nominated for the Nobel Peace Prize in 2014.