Second, a commitment to impartiality requires that one agree to consider evidence relevant to the charges, including new evidence that both House managers and White House defense lawyers seek to introduce.

Third, impartiality implies that senators will look to the chief justice to rule on various matters, including whether proposed evidence is relevant to the proceedings, and to listen to the reasons for his rulings. Impartial senators would usually defer to his judgment, because the chief is presumed to be actually impartial (in the non-Trumpian sense of the word)—though of course the senators will still reserve the responsibility to challenge the chief’s rulings if they have good reasons to disagree with them.

Finally, impartiality means that senators can sincerely tell their constituents that they would make the same decision regarding their verdict if the president was a member of the opposite party. This means that Democratic senators who support impeachment could affirm that they would also support impeachment if, for example, Hillary Clinton or Barack Obama were charged with the same offenses and with similar evidence. And Republican senators who oppose impeachment could sincerely say that they would not seek to impeach a Democratic president charged with the same offenses with a similar body of evidence.