The Oath Act of June 1, 1789, was the first legislation passed by the Senate and the first law signed by President George Washington. It prescribed the following oath: “I __ do solemnly swear (or affirm) that I will support the Constitution of the United States.” On June 4, 1789, Senate president John Adams administered that new oath to all senators, setting a pattern that future presiding officers followed, without controversy, for the next 74 years.
During the Civil War, Congress instituted the Test Oath, created in part to prevent ex-Confederates from taking federal positions. The Senate amended this oath four times between 1864 and 1884, when as part of a general revision of its rules, it repealed the Test Oath. Since then, the solemn oath prescribed by law has been: “I 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.”