Russ Feingold, a Democrat, represented Wisconsin in the Senate from 1993 to 2011.

No one deciding to run for the U.S. Senate gives much thought to the Senate’s role in impeachment. After all, before its dramatic vote Wednesday, the House of Representatives had impeached a president only twice since the nation’s founding. But now, the Senate will conduct a trial to determine whether President Trump will be removed from office. No proceeding in the Senate is more rare, and, with the possible exception of voting on whether to declare war, no duty of a senator is more solemn or consequential.

The Constitution specifically provides that when the Senate is trying an impeachment, senators “shall be on Oath or Affirmation.” Senate rules adopted in 1868 for the trial of President Andrew Johnson prescribe the oath that all senators will take: “to do impartial justice according to the Constitution and laws.”

As a senator from Wisconsin, I took this same oath in 1999 for the impeachment trial of President Bill Clinton. Taking the oath seriously led me to break with my party and oppose a motion to dismiss that would have ended the trial before the House managers had a chance to present the evidence they deemed most important. I also voted to permit the House managers to take new testimony from central witnesses in the case, including Monica Lewinsky.

My view both then and now is that, because senators have a singular role in impeachment proceedings under the Constitution, they must treat their responsibilities with the utmost seriousness and care. This admonition applies equally to Democratic and Republican senators, regardless of how they may ultimately vote. That is why it was so troubling to hear the chairman of the Senate Judiciary Committee, Lindsey O. Graham (R-S.C.), say recently, “I’m not trying to pretend to be a fair juror,” and to hear Senate Majority Leader Mitch McConnell (R-Ky.) say, “We’ll be . . . working in total coordination with the White House Counsel’s office.”

It is most certainly true that the role of senators in an impeachment trial is not precisely to be “jurors.” Indeed, then-Supreme Court Chief Justice William H. Rehnquist ruled in the Clinton impeachment that “the Senate is not simply a jury; it is a court in this case.” Judges in court cases are not limited to the arguments made by the parties. They conduct their own research and evaluation of the applicable law. They issue rulings based on the totality of their experience and knowledge.

Senators can and should do the same. The Constitution gives senators alone the responsibility to determine whether an abuse of presidential power occurred, and whether it is serious enough to warrant removal from office. While the chief justice presides over the trial, he will not provide instructions for senators to follow in reaching their verdict. Nor must their decisions be made in a vacuum, insulated from history and consideration of consequences. Indeed, I believe it is entirely appropriate for senators to consider the impact on the country of removing a president duly elected by the American people through the democratic process.

But judges, like jurors, are required to be impartial, and that is what the Senate impeachment oath requires. Working hand in glove with one side or the other as the trial proceeds is not consistent with that oath. And it is highly inappropriate for a senator to declare in advance that he or she cannot or will not be impartial in an impeachment trial.

While Senate Republican leaders have been failing their constitutional responsibilities, their Democratic colleagues must remember to uphold them. As the trial approaches, Senate Democrats, including or perhaps especially those who are running for president, must be cautious in their comments. Many of them have expressed strong views about what the president has done and have urged the House to impeach the president. They are entitled as political leaders to express those views. But soon they will take the oath to do impartial justice. They must keep an open mind and consider the defense presented by the president and his attorneys before deciding whether to convict him.

Comments from Sen. Patrick J. Toomey (R-Pa.) this past weekend offered a reassuring contrast to those of his party’s leaders: “I think it would be extremely inappropriate to put a bullet in this thing immediately when it comes over. I think we ought to hear what the House impeachment managers have to say, give the president’s attorneys an opportunity to make the defense and then make a decision about whether, and to what extent, it would go forward from there.” All senators should heed his words.

Whatever one thinks of the Clinton impeachment 20 years ago, the trial in the Senate is generally seen as having been fair. How the Senate handles this impeachment trial will not only determine whether Trump completes his term, it will also set important guideposts for the future of this very rarely needed, but still crucial, constitutional remedy for abuse of power. Senators would do well to keep that in mind as they prepare for the upcoming trial. History will not look kindly upon them if they don’t.

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