Russ Feingold: Senators have a duty to keep an open mind on impeachment 

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.

Source: Russ Feingold: Senators have a duty to keep an open mind on impeachment – The Washington Post