The president pro tempore of the United States Senate (also president pro tem) is the second-highest-ranking official of the United States Senate. Article I, Section Three of the United States Constitution provides that the vice president of the United States, despite not being a senator, is the president of the Senate. It also establishes that the Senate must choose a president pro tempore to act in the absence of the vice president:
In practice, neither the vice president nor the president pro tempore usually presides; instead, the duty of presiding officer is rotated among junior senators of the majority party to give them experience in parliamentary procedure.
The president pro tempore is third in the line of succession to the presidency, after the vice president and the speaker of the House of Representatives and ahead of the secretary of state.
Since 1890, the president pro tempore has held the office continuously until the election of another president pro tempore, and since 1945 the most senior senator in the majority party has generally been chosen.