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.

Sen. Pete V. Domenici (R-N.M.) in 2001. (Ray Lustig/The Washington Post)

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, in 2008, that was said to have been a factor, along with his health concerns, in Mr. Domenici’s decision not to seek a seventh term. In April of that year, the Senate ethics committee reprimanded him for an “appearance of impropriety” stemming from his involvement in the George W. Bush administration’s firing of nine U.S. attorneys.

During that time, Mr. Domenici and Rep. Heather A. Wilson (R-N.M.) were alleged to have pressured David Iglesias, one of the attorneys asked in December 2006 to resign, to pursue corruption cases before the November elections. Mr. Domenici later admitted that he had called Iglesias about the case, but he denied pressuring him.

Pietro Vichi Domenici was born in Albuquerque on May 7, 1932, to an Italian immigrant mother and an Italian-born father who earned U.S. citizenship after World War I service. Pete — a name he legally took when he entered public life in the 1960s — was the youngest of six siblings and the only boy. He worked in his father’s thriving grocery business as a child.

He received a teaching degree from the University of New Mexico in 1954. He was a standout baseball player in college and subsequently pitched in the minor leagues for the Albuquerque Dukes.

He received a law degree from the University of Denver in 1958 and then co-founded an Albuquerque law practice. The same year, he married Nancy Burk, with whom he had eight children. Survivors include his wife, children and many grandchildren.

Mr. Domenici was elected to the Albuquerque City Commission in 1966 and the next year was named chairman, a post equivalent to mayor at the time. After an unsuccessful run in 1970 for a gubernatorial nomination, he sought an open Senate seat in 1972 and won.

Dodd said of his erstwhile colleague, “He thought his job was not necessarily to appear on news shows, but rather to be a workhorse senator who was able to persuade his colleagues of the value of what he was trying to do, and he did it with great feeling.”

Nowhere was that ability to persuade more on display than in the budget battles of the 1980s. He became Budget Committee chairman when Republicans took control of the Senate in the Ronald Reagan landslide of 1980 and held together a compromise $695 billion budget that cut programs in every area of the budget but defense.

“I’m not an advocate of a balanced budget every year, under all circumstances,” he said at the time. “I advocate it as a tool to force a sound fiscal policy and as a tool to run contra to the propensities and tendencies of the last 25 years to overspend.”

He was an early supporter of the Reagan-era “supply side” tax cuts aimed at stimulating economic growth, spearheaded by Rep. Jack Kemp (R-N.Y.) and Sen. William V. Roth Jr. (R-Del.). But Mr. Domenici questioned whether the tax cuts alone would ward off inflation while raising enough revenue to balance the budget. The Kemp-Roth legislation passed Congress and was signed by Reagan. Critics said the tax cuts worsened the deficit.

In 1995, Mr. Domenici championed legislation to end the federal budget deficit in seven years. President Bill Clinton signed that bill, which became the Balanced Budget Act of 1997. Going against a leader in his own party, Mr. Domenici opposed the “Contract With America” of then-House Speaker Newt Gingrich (R-Ga.) because it called for what he considered unrealistic tax cuts.

He lost bids for majority leader, among other positions, and was passed over in 1988 as presidential candidate George H.W. Bush’s running mate in favor of Dan Quayle, a youthful senator from Indiana. Mr. Domenici was dumbfounded by the selection, telling reporters, simply, “I’m not so sure it’s a very good process.”

After Mr. Domenici left the Senate, he and former White House budget director Alice Rivlin chaired a debt-reduction task force sponsored by the Bipartisan Policy Center in Washington.

Mr. Domenici emerged as an unexpectedly impassioned voice during a Senate debate in 2006 on an ultimately unsuccessful attempt at immigration reform, when many in his party were portraying immigrants as criminals or major burdens on society.

He recalled an incident during World War II, when his mother, then an illegal immigrant, was swept up in a raid aimed at Italian “sympathizers.”

“I understand this whole idea of a household with a father who is American and a mother who is not, but they are living, working and getting ahead,” he said. “I understand that they are just like every other family in America. There is nothing different. They have the same love, same hope, same will and same aspirations as those of us who were born here have.”