The Constitution sets three qualifications for service in the U.S. Senate: age (at least thirty years of age); U.S. citizenship (at least nine years); and residency in the state a senator represents at time of election. The details of these qualifications were hammered out by the Constitution's framers during the Constitutional Convention in 1787.
Article I, section 3 of the Constitution requires the Senate to be divided into three classes for purposes of elections. Senators are elected to six-year terms, and every two years the members of one class--approximately one-third of the senators--face election or reelection.
The Constitution does not provide an oath of office for members of Congress, but specifies only that they "shall be bound by Oath of Affirmation to support this constitution." The oath of office that one-third of the Senate recites every two years is a product of the 1860s, drafted by Civil War-era members intent on ensnaring traitors. The oath-taking, however, dates back to the First Congress in 1789. The first oath served the Senate for nearly three-quarters of a century. The current oath, in use since 1884, is a milder version of the oath adopted in 1862.