Centrist Republican Richard Lugar died on Sunday at age 87 reports Reuters.
He championed nuclear nonproliferation during his 36 years in the U.S. Senate.
The Lugar Center, a Washington-based nonprofit, said in a statement that he died peacefully.
He was at the Inova Fairfax Heart and Vascular Institute due to complications from CIDP.
CIDP is chronic inflammatory demyelinating polyneuropathy, a rare neurological disorder.
Lugar was a professorial Midwesterner known for his keen intellect and mild demeanor.
He served 8 years as mayor of Indianapolis before his long stint in the Senate from 1977 to 2013.
He was the longest-serving senator ever from Indiana.