interesting, but a little error...
The Democratic ticket of Governor Bill Clinton and Senator Sam Nunn defeat ..in 1992.
... Vice President Nunn ...re-elected in 1996...
Sam Nunn has by 2000 ... served in the U.S. Senate for 24 years, 1972-1996.
You're right. Being elected as Vice President in 1992 would then mean Sam Nunn had served 20 years in the Senate, 1972-1992. (actually 1973-1993 from being sworn in to the Senate to being sworn in as Vice President).
Thank you.