James Flug, a Washington lawyer and hard-driving aide to Sen. Edward M. Kennedy who helped defeat two of Nixon’s Supreme Court nominees and spearheaded the first Senate investigation into the Watergate scandal, died Dec. 9 at his home in the District. He was 81.

The cause was lymphoma, said his daughter Amanda Flug Davidoff.

Mr. Flug came to Washington in 1963, clerking for a federal appeals court judge after graduating from Harvard Law School. He spent his career enmeshed in national politics, working as chief counsel to Kennedy (D-Mass.) on the Judiciary Committee; executive director of the National Legal Aid & Defender Association; head of Energy Action, a public-interest group that battled the oil industry; and as Washington counsel to generic drugmakers.

“He was very much part of that cresting wave of young people who were so profoundly influenced by [John F.] Kennedy, and the notion of coming to Washington and being part of the government,” said his friend Steven V. Roberts, a veteran political journalist who once likened the energetic Mr. Flug to “a bowling ball roaring down the alley.”

Mr. Flug returned to the Senate for a curtain call in the early 2000s, advising Kennedy during the Supreme Court confirmation hearings for John G. Roberts Jr. and Samuel A. Alito Jr. By then he had acquired a reputation as a formidable investigator and nemesis of Senate conservatives, having helped torpedo President Richard M. Nixon’s nominations of Clement F. Haynsworth Jr. and G. Harrold Carswell more than three decades earlier.

“Jim Flug is the 8,000-pound gorilla who will be driving this thing,” Makan Delrahim, a former Judiciary Committee counsel under Sen. Orrin G. Hatch (R-Utah), told the Washington Times in 2005, soon after Roberts was nominated.

Mr. Flug’s investigations failed to unearth major controversy, or at least enough to block the appointments. He had more success helping the Democrats defeat Haynsworth, a Southerner whose 1969 nomination to the Supreme Court was rejected amid accusations that the federal appeals court judge was “antilabor” and a “laundered segregationist.”

Arguably more significant was Mr. Flug’s role in leading the fight against Carswell, Nixon’s second choice for the vacancy, who also faced criticism for his civil rights record. “The Senate didn’t really want to go to war a second time,” said Nixon biographer John A. Farrell. “Over time, Flug’s digging and leading participation with [lawyer] Marian Wright Edelman, the leadership conference on civil rights, the legal community and other liberal Senate aides and senators turned the tide.”