J. Bennett Johnston Jr., a Louisiana Democrat and four-term United States senator who helped shape America’s energy and science policies in an era of rising concerns over the perils of nuclear power and the nation’s dependence on foreign oil, died on Tuesday in Arlington, Va. He was 92.

His death was confirmed by his son J. Bennett Johnston III.

One of a new breed of polished Southern Democrats that included Presidents Jimmy Carter and Bill Clinton, Mr. Johnston served in the Senate from 1972 to 1997, a tenure that included Middle East conflicts that threatened American oil imports, and nuclear licensing and safety changes in the aftermath of the nation’s worst nuclear accident, the partial reactor meltdown at Three Mile Island in Pennsylvania in 1979.

A target of environmentalists’ wrath, he favored more nuclear power plants, although public safety concerns limited new construction for decades. But he won fights to sharply expand oil drilling in the Gulf of Mexico, the major offshore petroleum-producing area for the United States, and sponsored laws to let coastal states share federal revenue from offshore drilling.

As chairman or a ranking member of the energy and natural resources committee from 1973 to 1996, he was involved in virtually all Senate energy legislation, from rewriting the nuclear licensing provisions of federal law to developing synthetic fuels and deregulating oil and natural gas prices to spur production. It was a delicate balancing act for a senator from a state with ferociously competing energy interests.

In a state also renowned for flamboyant politicians like Huey and Earl Long and corrupt rogues like former Gov. Edwin W. Edwards, Mr. Johnston was a notable exception — a quiet intellectual with finely honed political judgments who grasped the technical intricacies of energy exploration and production and could also lucidly discuss astrophysics, subatomic particles and tennis serves.

A trim, athletic man with receding hair, Mr. Johnston — an inveterate apple muncher who was said to be the Senate’s most avid tennis player in his 50s — was an approachable, friendly man, responsive to questions and easy to talk to or negotiate with.

His voting was not based on loyalties. Colleagues said he switched sides according to his views on the merits of proposed legislation. He advocated higher gas-mileage standards for auto manufacturers, but opposed President Ronald Reagan’s strategic defense initiative — a plan to use weapons in space to protect America from nuclear attack — calling it ill-conceived and too costly.

On international policy, he often sided with liberals in support of the United Nations and foreign aid. But he joined conservatives in opposing abortion and most gun-control measures, and championed a 1981 bill to limit busing for racial integration in public schools to five miles or 15 minutes. The measure died in the House of Representatives.

In Senate fights over candidates for the Supreme Court, Mr. Johnston helped lead a 1987 rejection of Robert H. Bork as President Reagan’s nominee, but broke with his party in 1991 to support confirmation of President George H.W. Bush’s nominee, Clarence Thomas.

In 1988, with Democrats in control of the Senate and Robert F. Byrd of West Virginia stepping down as their leader after a decade, Mr. Johnston and Senator Daniel K. Inouye of Hawaii ran for majority leader, the Senate’s most powerful post. Both lost to Senator George J. Mitchell of Maine.

Mr. Johnston’s support for higher education landed $110 million for five national research centers at universities in Louisiana. He crusaded for years for billions for the Superconducting Super Collider, a pure research particle accelerator, in Texas, to search for fleeting subatomic structures. “It was lynched by the know-nothings,” he said when the project was canceled in 1993.

“I’m interested in understanding where the universe came from and where it’s going,” Mr. Johnston told Physics Today magazine in 1996. “I’m interested in the Higgs boson, which high-energy physicists hope to find if it exists at all, and, like them, I also hope the search produces surprises.” (In 2012, scientists announced that they had discovered a new subatomic particle that appeared to be the Higgs boson.)

John Bennett Johnston Jr., who rarely used his first name, was born in Shreveport, La., on June 10, 1932, to John Bennett Johnston Sr., a lawyer, and the former Wilma Lyon. He graduated from Shreveport schools and attended the United States Military Academy at West Point and Washington and Lee University before graduating from law school at Louisiana State University in 1956.

He married Mary Gunn the same year. They had four children: J. Bennett Johnston III, Hunter Johnston, Mary Johnston Norriss and Sally Roemer.

In the Army from 1956 to 1959, he became a first lieutenant with the Judge Advocate General’s Corps in Germany. After practicing law in Shreveport for several years, he began his political career in 1964 with election to the Louisiana House of Representatives. In 1968 he won a four-year term in the State Senate.

In a state dominated by Democrats, with nominations tantamount to election, Mr. Johnston in 1971 ran for governor, but narrowly lost the nomination to Representative Edwin Edwards, who then won the first of his four terms as governor. Mr. Edwards later went to jail for eight years for bribery and extortion. In 1972, Mr. Johnston contested the renomination of United States Senator Allen J. Ellender, who had held his seat since 1936 as a protégé of the assassinated Senator Huey P. Long.

But Mr. Ellender died during the campaign. Mr. Edwards named his own wife to the seat pending a special election, and Mr. Johnston won the nomination and the general election. He was re-elected in 1978 and again in 1984 against token opposition, despite a landslide for President Reagan that hurt other Democrats.

Mr. Johnston’s last campaign, in 1990, was his toughest — against David Duke, a former Ku Klux Klan leader who had become a popular state legislator. Even by Louisiana’s baroque political standards, the race was strange: a powerful three-term Democratic incumbent overshadowed by a political neophyte who had not sponsored a single bill in the Louisiana Legislature.

Mr. Duke dominated the campaign with appeals to white resentment over affirmative action and welfare programs, and allusions to his racially charged agenda. But his candidacy and his past associations with white supremacy groups were widely condemned, and Mr. Johnston won a fourth term.

When that term ended in January 1997, Mr. Johnston, who lived in McLean, Va., retired from politics and founded Johnston & Associates, a Washington a lobbying firm that later went out of business.

Mr. Johnston’s son said that he is survived by his wife, his four children and 10 grandchildren.

Yan Zhuang contributed reporting.