In a normal trial the judge presides over jury selection during the voir dire process. Jurors are excused for causes such as lack of impartiality. Does the Chief Justice of the US Supreme Court have the authority to excuse senators from a presidential impeachment if the Chief Justice determines them to be partial to one side or the other?
Can The Chief Justice Excuse Senators From The Jury In A Presidential Impeachment?
No, the jury (such as it may be) is fixed by the Constitution. Article I, Section 3, "The Senate shall have the sole Power to try all Impeachments."
Article 2, Section 4, "The President, Vice President and all Civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors."
Trials of impeachment may include judges and other civil officers, where the chief justice would not be the presiding officer. There is nothing in the Senate rules permitting the presiding officer to excuse any Senator. However, a Senator may request to be excused, by stating a reason, and by vote of the majority.1
1 PROCEDURE AND GUIDELINES FOR IMPEACHMENT TRIALS IN THE UNITED STATES SENATE, Excused from Participation in Trial or from Voting, pp. 77-78.