The president pro tempore ( or ), also president pro tem, is the second-highest-ranking official of the United States Senate. According to the United States Constitution, the Vice President of the United States is the President of the Senate, despite not being a senator, and the Senate must choose a president pro tempore to act in his absence. Since 1890, the most senior senator in the majority party has generally been chosen to be president pro tempore; this tradition has been observed without interruption since 1949.