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If you know anything about the Constitution, you know Republicans have no choice — they have to subpoena John Bolton

At the start of President Trump’s impeachment trial, Chief Justice John Roberts administered the following oath to each Senator present: “I do solemnly swear (or affirm, as the case may be) that in all things appertaining to the trial of the impeachment of Donald John Trump, now pending, I will do impartial justice, according to the Constitution and laws: [So help me God.]”

Taking this oath of impartiality is not just a matter of Senate custom. It is required by the Constitution itself, which states in Article I, Section 3, Subsection 6: “The Senate shall have the sole Power to try all Impeachments. When sitting for that Purpose, they shall be on Oath or Affirmation.”

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This oath is in addition to the standard oath “to support and defend the Constitution of the United States against all enemies, foreign and domestic“ that Senators must take when they assume office. This special impeachment oath is the only oath (beyond the standard oath of office) mentioned in the Constitution.

The US Constitution is not a wordy text. It can fit in one’s pocket. Its often open-ended language leaves many fundamental questions unanswered (“What does ‘cruel or unusual punishment’ mean, exactly?”) The Framers did not say a lot about impeachment — a mere 150 words in four sections. They left the language “or other high Crimes and Misdemeanors” deliberately vague, for fear that a future tyrant or demagogue could abuse the power of his office in unforeseen ways. But the Framers spoke with specificity when they included the oath of impartiality, and Senators today, like Senators during the impeachment trials of Presidents Johnson and Clinton, are not free to ignore it.

The New York Times reported Sunday that former White House National Security Adviser John Bolton’s memoir, scheduled to be published in March, discusses his conversations with President Trump regarding the withholding of military assistance to Ukraine. Bolton’s testimony would go to the heart of the Articles of Impeachment. He has said that he is willing to testify in President Trump’s impeachment trial, if subpoenaed by the Senate.

This places Senate Republican leadership in an awkward position, to say the least. Some Senators have insisted that they have heard nothing new in the impeachment trial. Some argue that they are limited to the evidence provided in the proceedings in the House -- a specious argument, given that the White House expressly forbade government employees from testifying before the House.

More recently, President Trump and some Republicans have tried to impinge Bolton’s credibility, which they are absolutely free to do — in the Senate trial, after Bolton has had the opportunity to testify. But it defies logic to take shots at the credibility of the former White House national security council adviser before he is given the opportunity to testify under oath.

No one knows for sure whether John Bolton’s testimony will be enough to persuade two thirds of Senators to convict President Trump. But it is impossible to reconcile a vote against subpoenaing Bolton with the oath of impartiality. Republicans and Democrats might disagree about how much weight to give his testimony, or whether the conduct alleged meets the Constitutional standard. But they cannot disagree about basic norms of procedural fairness, in light of the oath of impartiality.

Some questions of law and policy are genuinely difficult. Reasonable people can — and do —  disagree about questions of substantive justice, such as: “How much environmental regulation is appropriate?” or “What should the corporate tax rate be?” But Americans do not disagree about what “impartiality” means. It is a value we teach our children as early as preschool, even if we do not always honor it in practice. It is the foundation of our legal system.

Every day, we ask, and expect, ordinary Americans to administer impartial justice when they serve on juries. If impartial justice is something we ask of jurors, it is reasonable to demand it of our Senators, whether they are Republicans or Democrats. And despite the hyper-polarized world that we live in, it is still reasonable for American citizens to demand that those whom we have elected to represent us remember the meaning of an oath.

Alexandra Dufresne is a legal expert who taught law and policy at Yale and now teaches in Switzerland. She is currently writing a book about US law