Domenici was subsequently re-elected in 1978, 1984, 1990, 1996, and 2002 and is to date the longest-serving senator in his state's history, having served in the legislative body for 36 years. At the time of his retirement, he was the ranking member of the United States Senate Committee on Energy and Natural Resources and the United States Senate Appropriations Subcommittee on Energy and Water Development. He was also a member of the U.S. Senate Committees on Appropriations and Indian Affairs, and served as Chairman and Ranking Minority Member of the Budget Committee. He advocated for the mentally ill, having pushed the Mental Health Parity Act of 1996.