Answer:
The President Pro Tempore of the United States Senate is the second most important position in the Senate and the highest position as senator. It is established by the First Article of the Constitution of the United States. The vice president of the United States is the ex officio president of the Senate, this being the highest position of a member of that body; during his absence, the president pro tempore chairs the Senate.
It is elected by the Senate; by custom it is, typically, the senior senator of the majority party. The president pro tempore is the third person in succession to president of the United States, after the vice president of the United States and the president of the House of Representatives of the United States.
The current president pro tempore of the Senate is Chuck Grassley, from the Republican Party.