Idaho Code § 19-4009: Senate to be sworn.

At the time and place appointed and before the senate proceeds to act on the impeachment, the secretary must administer to the president of the senate, and the president of the senate to each of the members of the senate then present, an oath truly and impartially to hear, try, and determine the impeachment; and no member of the senate can act or vote upon the impeachment or upon any question arising thereon without having taken such oath.