Senate Majority Leader Mitch McConnell on Capitol Hill on Dec. 17. (Melina Mara/The Washington Post)

Lawrence Lessig is a professor at Harvard Law School and the author of “Fidelity and Constraint: How the Supreme Court Has Read the American Constitution.”

Before the Senate begins its trial to determine whether the president should be convicted of the charges for which he has been impeached, the jury — the members of the Senate — must be sworn to service. The oath is mandated by the Constitution; its language, set by Senate rules, requires each senator to swear to “do impartial justice according to the Constitution and laws.”

To swear a false oath is perjury — the crime President Bill Clinton was charged with in his impeachment. Yet given the Constitution’s speech or debate clause, a senator likely could not be charged with perjury for swearing falsely on the Senate floor. Instead, it is the Senate itself that must police members’ oaths — as it has in the past. Beginning in 1864, and continuing for 20 years, members had to swear an oath affirming their commitment to the Union. Often when it was clear that a member could not swear that oath honestly, he was not permitted to take it. As Massachusetts Sen. Charles Sumner said, “A false oath, taken with our knowledge, would compromise the Senate. We who consent will become parties to the falsehood.”

As Sumner saw it, this principle extended to any truth meant to be affirmed by a member’s oath of office. Under the Constitution, for example, a senator must be 30 years of age, nine years a citizen and a resident of the state from which they are elected. If, as Sumner explained, they do not meet those requirements, they “cannot be allowed to go to that desk” — meaning they could not be allowed to take the oath.

Among the senators who will have to take an oath in the trial of President Trump is the majority leader, Sen. Mitch McConnell (R-Ky.). Yet McConnell has openly declared that he is “not impartial about this at all.” “Impeachment,” the senator has opined, is a “political process. This [sic] is not anything judicial about it.”

But however one characterizes the process of impeachment, an oath is an oath. Even a majority leader — like a president — is not privileged to swear an oath falsely. And whereas most politicians are careful to avoid language that expressly declares the opposite of their pledge, McConnell has openly flouted the Constitution’s clear command to “do impartial justice.” If others follow, it would corrupt the Senate’s role in fairly adjudicating the charges.

Rep. Jackie Speier (D-Calif.) has called on McConnell to recuse himself. But the issue is far more serious. The real question is for the chief justice, who presides over the president’s trial: Can he accept an oath that he knows is false? Can he seat a juror who he knows has pledged not to be impartial?

During the impeachment of President Andrew Johnson, this question became a serious issue. Sen. Thomas Hendricks objected to swearing in the Senate’s president pro tempore, Benjamin Wade, because under the rules of succession, if Johnson were convicted, Wade would have become president — casting doubt on Wade’s pledge of impartiality. That objection threw the Senate into a fierce debate until, for reasons unknown, Hendricks withdrew his objection.

That precedent should matter today. Any senator is privileged to object to any other senator taking an oath. The chief justice would then have to decide whether the oath can be sworn honestly. As there seems no way that McConnell’s oath could be honest, Chief Justice John G. Roberts Jr. should forbid McConnell from taking it. Whether he so rules or not, the decision could be appealed to the Senate as a whole. Should the Senate openly accept a false oath — perjury — in a proceeding to determine the president’s guilt?

This would not be an easy question for many Republican senators. The strong distinction that they all have drawn between Trump’s case and Clinton’s is perjury: Clinton swore a false oath, and thus had to be impeached. But then how can those same senators accept the perjurious oath of another senator? Is the president forbidden from lying under oath but McConnell not?

Our cynicism about politicians makes it difficult for many to even see the issue. Whatever senators say, many citizens believe most have made up their minds. But believing that is fundamentally different from accepting an open declaration that a senator will not be impartial. A nation governed by the rule of law cannot accept a plain flouting of the law. Given his words, McConnell cannot honestly be sworn to serve on the president’s jury. Any senator who allows it becomes, in Sumner’s words, a party to that falsehood.