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.
At the start of each new Congress, in January of every odd-numbered year, one-third of senators take the oath of office to begin their new terms. While the oath-taking practice dates back to the First Congress in 1789, the current oath is a product of the 1860s, drafted during the Civil War.
The Constitution contains an oath of office for the president of the United States. 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 adopted a simple oath: "I do solemnly swear (or affirm) that I will support the Constitution of the United States."
At the outbreak of the Civil War 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. At the conclusion of its emergency session that summer, Congress adopted legislation requiring executive branch employees to take the expanded oath in support of the Union. In July 1862 Congress added a new section to the oath, which became known as the "Ironclad Test Oath." The Test Oath required civilian and military officials to swear or affirm that they had never aided or encouraged “persons engaged in armed hostility” against the United States. Government employees who swore falsely would be prosecuted for perjury and forever denied federal employment. Congress also revised the rest of the oath with language that closely resembles the modern oath.
Although Congress did not extend coverage of the Ironclad Test Oath to its own members, many took it voluntarily. At the urging of Senator Charles Sumner of Massachusetts, the Senate adopted a resolution in January 1864 to require all senators to take the Test Oath. The resolution also required senators to "subscribe" to the oath by signing a printed copy. This condition reflected a wartime practice in which military and civilian authorities required anyone wishing to do business with the federal government to sign a copy of the Test Oath. The current practice of newly sworn senators signing individual pages in an elegantly bound oath book dates from this period.
Following the Civil War, Congress permitted some former Confederates to take only the second section of the 1862 oath, and an 1868 statute prescribed this alternative oath for "any person who has participated in the late rebellion, and from whom all legal disabilities arising therefrom have been removed by act of Congress." Northerners complained of the law's unfair double standard that required loyal Unionists to take the Test Oath's harsh first section while permitting ex-Confederates to ignore it. In 1884, after more than a decade of such complaints, a new generation of lawmakers repealed the first section of the Test Oath, leaving intact today's affirmation of constitutional allegiance.