Who is the longest-serving U.S. Senator in history?

Robert Byrd (born Cornelius Calvin Sale Jr.; November 20, 1917 – June 28, 2010) was an American politician and musician who served as a United States senator from West Virginia for over 51 years, from 1959 until his death in 2010. A Democrat, Byrd also served as a U.S. representative for six years, from 1953 until 1959. He remains the longest-serving U.S. Senator in history; he was the longest-serving member in the history of the United States Congress until surpassed by Representative John Dingell of Michigan. Byrd is the only West Virginian to have served in both chambers of the state legislature and in both chambers of Congress.

Source: Wikipedia