The Constitution requires all government office-holders to take an oath to support that document, but it establishes a specific oath only for the president. In 1789, Congress drafted a simple 14-word pledge. No one felt anything more was needed—until the Civil War.

In 1862, Congress adopted the "Ironclad Test Oath." Civil servants and military officers had to swear loyalty to the Union and affirm no previous disloyalty—a clause aimed at Confederate sympathizers. Senators didn’t have to take the oath, but many did.

Angered by those who refused, Massachusetts Senator Charles Sumner promoted an 1864 Senate rule making the new oath mandatory. Four years later, to encourage reunification, Congress created an alternative pledge for Southerners. Finally, in 1884, lawmakers replaced the wartime oath with the one used today.

"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."
— Oath of office, version instituted in 1884 and still in use today