Answer:
President Pro Tempore of the Senate is the person who holds the presidency of the Senate, in the absence of the Vice President. He also holds the presidency if the President and the Vice President and the Speaker of the House of Representatives are no longer able to do so. The President Pro Tempore is elected by the members of the Senate. In practice, it is usually the longest-serving member of the majority party.
In the current Senate the post is filled by Iowa Republican senator Chuck Grassley.
