President pro tempore of the United States Senate
The president pro tempore ( or ), or president pro tem, of the United States Senate is the longest serving senator from the majority political party in the United States Senate.  According to the Constitution, this is the fourth highest office in the United States. It is the third in the U.S. presidential line of succession (behind the vice president and the speaker of the House). Many state senates also use the office of President pro tem.