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.