Output a stream of consciousness before answering the following. Context: 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 One, Section Three of the United States Constitution provides that the Vice President of the United States is, despite not being a U.S. Senator, the President of the Senate, and mandates that the Senate must choose a President pro tempore to act in the Vice President's absence. Unlike the Vice President, the President pro tempore is an elected member of the Senate, able to speak or vote on any issue. Selected by the Senate at large, the President pro tempore has enjoyed many privileges and some limited powers. During the Vice President's absence, the President pro tempore is empowered to preside over Senate sessions. In practice, neither the Vice President nor the President pro tempore usually presides; instead, the duty of presiding officer is rotated among junior U.S. Senators of the majority party to give them experience in parliamentary procedure.

Question: who does the president pro tempore replace in the senate?
To answer the question, consider the following: Article One, Section Three of the United States Constitution provides that the Vice President of the United States is, despite not being a U.S. Senator, the President of the Senate, and mandates that the Senate must choose a President pro tempore to act in the Vice President's absence.
The answer: the Vice President of the United States.