Who was the longest-serving senator in US history, serving from 1959 to 2010?

Robert Byrd is considered the longest-serving senator in US history because he served from 1959 until his death in 2010. Over his remarkable career, Byrd held various leadership positions, including serving as the President pro tempore of the Senate multiple times. Byrd was known for his deep knowledge of the Senate's rules and procedures, which he utilized to advocate for the interests of his home state, West Virginia. Despite controversies surrounding his early involvement with the Ku Klux Klan, Byrd evolved to become one of Congress's most respected legislators, leaving a significant impact on American politics throughout his long tenure.