The Constitution requires the Senate to elect a president pro tempore to serve as presiding officer in the absence of the vice president. The president pro tempore is authorized to preside over the Senate, sign legislation, and issue the oath of office to new senators.

For many years, the vice president routinely presided over the Senate, and presidents pro tempore were elected to serve only during the absence of the vice president. To give the office continuity, in 1890 the Senate established continuous terms for the president pro tempore. In his or her absence, the president pro tempore names other senators to perform the duties of the chair, allowing them to grow more accustomed to the Senate's rules and procedures. In the Senate's earlier years, it elected to the post senior members who had shown a particular knowledge of Senate rules and procedures. Since the mid-20th century, tradition has dictated that the position go to the senior member of the majority party. The Senate's president pro tempore also stands third in the line of presidential succession.

To learn more about the president pro tempore, the vice president, or other Senate officers, visit the Virtual Reference Desk, or read the new publication from the Senate Historical Office, Pro Tem.