Sen. Pete V. Domenici (R-N.M.) in 2005. (Melina Mara/The Washington Post)

Pete V. Domenici, a Republican lawmaker from New Mexico who became a leading voice on budget and energy policy during six terms in the Senate, and who a colleague once diagnosed as suffering from a “case of terminal responsibility,” died Sept. 13 at a hospital in Albuquerque. He was 85.

The death was announced by the law firm of his son Pete Domenici Jr. The senator had abdominal surgery in recent weeks and been treated for several years for frontotemporal lobar degeneration, a brain disease.

Mr. Domenici (pronounced doe-MEN-ih-chee), who represented his state in Congress from 1973 to 2009, was the longest-serving senator in New Mexico history. His seniority on the Appropriations Committee added to his power and prominence, but he was best known for his chairmanship of the Budget Committee during the turbulent budget and tax wars of the 1980s and ’90s.

A conservative with a pragmatic streak, he worked with Democrats to forge budget-cutting legislation. Then-Rep. Lynn Morley Martin (R-Ill.), citing Mr. Domenici’s deeply held convictions about the need for a balanced budget, reportedly issued the quip about “terminal responsibility.”

Mr. Domenici, who also chaired the Energy and Natural Resources Committee, was a champion of nuclear energy, pressing for legislation and federal money to support his state’s national laboratories and military installations. On the Indian Affairs Committee, he worked to set aside money for the settlement of Indian water disputes and land claims.

For decades, Mr. Domenici pursued mental health parity in health insurance coverage — an issue of personal importance to him, as one of his children was diagnosed with schizophrenia.

His chief ally was Sen. Paul Wellstone (D-Minn.), whose brother had suffered from mental illness. After Wellstone died in a plane crash in 2002, Sen. Edward M. Kennedy (D-Mass.) took up the cause of the bipartisan legislation.

By the time the bill finally passed in 2008 as part of the Troubled Asset Relief Program (TARP) bill that extended a lifeline to banks as well as insurance firms and auto companies, Mr. Domenici had decided not to run for another term.

“It never would have happened without Pete,” said former senator Christopher J. Dodd (D-Conn.), who was instrumental in incorporating the mental health bill into TARP in his post as chairman of the Senate banking committee. “He brought the Republican side to the issue because of his passion.”

Mr. Domenici’s reputation as a steady, unflashy senator — he was sometimes known as “Saint Pete” — was jarred by the revelation in 2013 that he had fathered a child in the 1970s with Michelle Laxalt, the daughter of fellow senator Paul Laxalt (R-Nev.). Michelle Laxalt, who became a lobbyist, was 24 at the time; Mr. Domenici was 46 and married.

Mr. Domenici, who had left office by the time of the disclosure, and his wife of more than 50 years, Nancy, held their family together. He and Michelle Laxalt said they revealed the affair because they suspected that someone else was about to go public with the information. Their son, Adam Laxalt, became Nevada attorney general in 2015.

It was another scandal,…