Article I, Section 3, Clause 6:
The Senate shall have the sole Power to try all Impeachments. When sitting for that Purpose, they shall be on Oath or Affirmation. When the President of the United States is tried, the Chief Justice shall preside: And no Person shall be convicted without the Concurrence of two thirds of the Members present.
The Constitution requires Senators sitting as an impeachment tribunal to take a special oath distinct from the oath of office that all Members of Congress must take. 1 This requirement underscores the unique nature of the role the Senate plays in impeachment trials, at least in comparison to its normal deliberative functions. 2 The Senate practice has been to require each Senator to swear or affirm that he will
do impartial justice according to the Constitution and laws. 3 The oath was originally adopted by the Senate before proceedings in the impeachment of Senator Blount in 1798 and has remained largely unchanged since. 4