DIED. Lawrence O'Brien, 73, veteran Democratic wheeler-dealer, John F. Kennedy's congressional liaison, Postmaster General under Lyndon Johnson and commissioner of the National Basketball Association from 1975 to 1984; of cancer; in New York City. Dubbed by Kennedy the "best election man in the business," O'Brien helped perfect the use of grass-roots voter registration drives, political teas, youthful volunteer workers and telephone banks, which have become staples of campaigning. During the second of O'Brien's two reigns as Democratic National Committee chairman, in 1972, men broke into his offices at Washington's Watergate complex. The subsequent scandal brought down Richard Nixon's presidency.