There are senators with longer records. As individuals. 6 of 'em in the current Senate. Hell, Rep John D. Dingell (D-Mich) has been constantly in the House since 1955, which is, in fact, before I was born. 16 Reps in the 113th Congress have more seniority than the Clinton hybrid.
2. And Patrick Leahy (D-VT) is the longest serving Senator.
Biden would have been but he left for the Vice Presidency. They won election young at 29 and 35.
Danny Inyoue was there for 49 years and Robert Byrd for 50. And Dingell was there since Ike was President. 1955, my parents were in elementary school!