Many posts have discussed oaths of office.
The Presiding Officer of the trial takes the oath of office. The Constitution requires that Senators be “on Oath or Affirmation” when sitting for the purpose of trying an impeachment. The Senate developed the practice of first swearing in the presiding officer of the trial, who then administers the oath to all Senators. In the case of a presidential impeachment, the Chief Justice acts as presiding officer. Impeachment Rule IV requires that notice be given to the Chief Justice of the time and place of the trial. It further provides that the Chief Justice is to be administered the oath by the “Presiding Officer of the Senate.” The Chief Justice takes the same oath as the Senators (see below for text). Although the Vice President of the United States, as President of the Senate, could act as Presiding Officer of the Senate and administer the oath to the Chief Justice, in the Clinton impeachment trial, the President Pro Tempore of the Senate administered the oath to the Chief Justice. In the Clinton trial the Senate also agreed by unanimous consent that a bipartisan group of six Senators escort the Chief Justice to the dais.
Senators are administered the oath of office. The Presiding Officer of the Trial administers the following oath to Senators, as provided in Impeachment Rule XXV:
[Do you] solemnly swear (or affirm, as the case may be) that in all things appertaining to the trial of the impeachment of____, now pending, [you] will do impartial justice according to the Constitution and laws: So help [you] God.
In modern practice, the Chief Justice asks all Senators, who are standing at their desks, to raise their right hands as he reads the oath, and Senators respond, all together, “I do.” Senators also sign an official oath book, which serves as the permanent record of the administration of the oath. Senators are required to take the oath before participating in the trial, and Senators who might be absent at the time the oath is administered en masse inform the presiding officer as soon as possible so that they can take the oath separately. At this point, any Senator wishing to be excused from participating in the trial could ask to be excused from this service. In the past, the Senate has excused Senators from service in an impeachment trial only at their request.