Employees of the United States Government including all members of Congress are required to take the following oath before assuming elected or appointed office.
5 U.S.C. 3331:
“An individual, except the President, elected or appointed to an office of honor or profit in the civil service or uniformed services shall take the following oath: ‘I, AB, do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservation or purpose of evasion; and that I will well and faithfully discharge the duties of the office on which I am about to enter. So help me God.’”