J. Fred Buzhardt Jr., 54, a politically conservative military officer turned lawyer who was one of former president Richard Nixon's key legal defenders during the Watergate erar, died yesterday in a South Carolina hospital after a heart attack.

Quiet, intense and extremely hardworking, Mr. Buzhardt was stricken at his home in Beaufort and died shorlty after being taken to Hilton Head Hospital. He had practiced law since 1975 in both Beaufort and the nearby resort communicty of Hilton Head Island.

Named special White House counsel for Watergate matters on May 10, 1973, Mr. Buzhardt served as the president's top legal defender during the period of the Senate Watergate hearings and through much of the legal controversy over the celebrated White House tapes.

In a telephone interview on June 30, 1973, after the testimony of former White House counsel John W. Dean, Mr. Buzhardt gave a strong impression of bewilderment.

Dean had alleged before the Senate committee that Nixon was involved in the cover-up.

"What do you do?" adked Mr. Buzhardt. "Do you (have the president) testify or make a statement after each witness?"

He also said he did not know which one if any of the president's former aides to believe. "I'm not sure what I believe beyond the innocence of the president," he said.

The interview came after Mr. Buzhardt had given a controversial memo to the Watergate committee calling Dean the "principal actor in the Watergate cover-up," and placing much of the blame for the scandal on former attorney general John N. Mitchell.

During the interview, the bespectacled attorney said his role in the White House was "not that of an investigator," but rather one that primarily ivolved supplying information and documents to the Senate committee and the special prosecutor's office.

After the revelation in July 1973 of the existence of the White House tapes, Mr. Buzhardt was active in resisting efforts by the special prosecutor's office to obtain them.

In this regard, it fell to Mr. Buzhardt to make known one of the most celebrated symbols of the entire Watergate affair-the famed, and as yet unexplained, 18 1/2-minute gap.

On Nov. 21, Mr. Buzhardt told Judge John J. Sirica of the U.S. District Court that an 18 1/2-minute portion of a key tape contained an "audible tone" but no conversation.

The tape in question was made during a conversation on June 20, 1972, between Nixon and his chief of staff, H.R. (Bob) Haldeman. The gap occurred at a point where discussion by the two men of the scnadal was just beginning.

After court testimony about the gap on Nov. 29, Mr. Buzhardt told reporters he still had "no explanation" for the gap, whose existence was seen as an important element in the undermining of Nixon's credibility.

On Jan. 4, 1974, it was announced that James D. St. Clair would handle the president's Watergate defense. Mr. Buzhardt was elevated to the post of White House counsel with the expectation, according to a White House spokesman, that he would participate occasionally in the defense, but would gradually become absorbed in other duties.

Mr. Buzhardt continued, according to a White House aide, to work seven days a week, 16 to 18 hours a day. At 12:30 a.m. June 13, he was admitted to Fairfax Hospital, suffering from a heart attack.

Mr. Buzhardt recovered and returned to work. On July 24, 1974, the Supreme Court ruled that Nixon could not withhold from prosecutors 64 disputed tapes. Soon afterward Nixon reportedly telephoned Mr. Buzhardt to say "there may be some problem with the June 23 tape."

Mr Buzhardt studied the tape, found the significant passages in a conversation between Nixon and Haldeman, and concluded, according to reconstructions of the last days of the Nixon administration, that the president could not remain in office.

Nixon resigned Aug. 9 and Mr. Buzhardt returned the next year to practice law in his native state.

Joseph Fred Buzhardt was born in Greenwood, S.C., on Feb. 21, 1924, and grew up in McCormick, S.C. During World War II he served in the Army Air Corps for two years, and in 1946 he graduated from West Point.

He was an Air Force first lieutenant and troop carrier pilot in 1950 when an inability of his ears to withstand the pressure of flying caused him to abandon plans for a military career.

The son of a lawyer, Mr. Buzhardt studied law at the University of South Carolina. He graduated in 1962 and practiced law in a rural section of his state with his father, a friend of Sen. Strom Thurmond (R.S.C.).

Mr. Buzhardt worked as an aide to Thurmond for eight years, specializing in military affairs and winning a reputation as an expert in the workings of the Pentagon bureaucracy.

He enhanced his reputation after joining the Defense Department in 1969 as special assistant to the assistant secretary of Defense. Named general counsel in 1970, he was known unofficially as the Pentagon's top troubleshooter, handling many of the thorny problems that confronted the military in the 1970s.

"He was one of the most capable and astute men I have ever known," Sen. Thurmond said in a statement. Mr. Buzhardt was married and had four children.