At the start of each new Congress, in January of every odd-numbered year, the entire House of Representatives and one-third of the Senate performs a solemn and festive constitutional rite that is as old as the Republic. While the oath-taking dates back to the First Congress in 1789, the current oath is a product of the 1860s, drafted by Civil War-era members of Congress intent on ensnaring traitors.

The Constitution contains an oath of office only for the president. For other officials, including members of Congress, that document specifies only that they "shall be bound by Oath or Affirmation to support this constitution." In 1789, the First Congress 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."

For nearly three-quarters of a century, that oath served nicely, although to the modern ear it sounds woefully incomplete. Missing are the soaring references to bearing "true faith and allegiance;" to taking "this obligation freely, without any mental reservation or purpose of evasion;" and to "well and faithfully" discharging the duties of the office.

The outbreak of the Civil War quickly transformed the routine act of oath-taking into one of enormous significance. In April of 1861, a time of uncertain and shifting loyalties, President Abraham Lincoln ordered all federal civilian employees within the executive branch to take an expanded oath. When Congress convened for a brief emergency session in July, members echoed the president's action by enacting legislation requiring employees to take the expanded oath in support of the Union. This oath is the earliest direct predecessor of the modern oath.