THE REPUBLICAN WAR ON THE CONSTITUTION:

The third oath is the rarest. In Article I, the Constitution gives the Senate the "sole" power to "try all impeachments," and the Constitution requires that "when sitting for that Purpose, they shall be on Oath or Affirmation." This special oath only kicks in when the Senate tries an impeachment, and this will be only the third time when a president has been so tried. The framers wanted to make sure the Senate would never take such a trial lightly -- this oath requirement is over and above the oath each senator has already taken to support the Constitution.

The Constitution does not set out the text of the trial oath, but the Senate rules do. Senators will ''solemnly swear ... that in all things appertaining to the trial of the impeachment of Donald J. Trump, now pending, I will do impartial justice according to the Constitution and laws: So help me God.''

The presidential oath and the senatorial oath to be taken before an impeachment trial are kin. The president must act faithfully and without corruption. In those (presumably) rare situations in which the president has failed to be faithful, the Senate is required to be faithful in its adjudication of the case against him.

But we have already seen indications that McConnell has no intention of doing impartial justice. He has said that he does not consider himself an "impartial juror." He is coordinating strategy with the White House. He has already called the case against the president "thin" and "incoherent."

Every senator has a constitutional obligation of impartiality. But McConnell's role as Senate leader makes his obligation even more important and crucial to the constitutional framework. This is not a time for political cynicism or constitutional faithlessness. McConnell's loyalty to Trump should not overwhelm his loyalty to the Constitution. If he fails in this, he is not only violating his Article I oath but his Article VI oath.

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