The Constitution contains an oath of office only for the president. For other government officials, including members of Congress, the document specifies only that they "shall be bound by Oath or Affirmation to support the Constitution."

The First Congress, in 1789, reworked this requirement into a simple fourteen-word oath: "I do solemnly swear (or affirm) that I will support the Constitution of the United States."

At the beginning of the Civil War, Congress expanded the oath. That text is the earliest direct predecessor of the current oath. Then, in August 1862, a section was added, known as the "Ironclad Test Oath."

It required all civil servants and military officers to swear not only to future loyalty, as required by the existing oath, but also to affirm that they had never previously engaged in criminal or disloyal conduct. Those failing to take the 1862 Test Oath would not receive a salary. Ironically, Congress did not extend coverage of the Ironclad Test Oath to its own members.

In 1884, two decades after the Civil War ended, Congress quietly repealed the Test Oath section, leaving intact what is today's current affirmation of constitutional allegiance:

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.

 The Senate's First Act--the Oath Act, May 5, 1789

Senator Resigns to Protest Loyalty Oath, January 29, 1864

Oath Taking

Term of a Senator: When Does It Begin and End? (pdf)