IF there is such a thing as a political legacy, it was evident in Tuesday's Democratic primary in Massachusetts' Eighth Congressional District. Young Joe Kennedy II, Robert F. Kennedy's eldest son, won the right to run for the seat from which his uncle, John F. Kennedy, began his political ascent. The Eighth has been the seat of Thomas P. O'Neill for all the 34 years since Jack left it for the Senate, and Tip is retiring as Speaker of the House at the end of this year -- quite a prominent record for a district that includes intellectual Cambridge and the blue-collar suburbs of Waltham and Watertown.
But legacies, in politics as elsewhere, provide only a start. Joe will have to make his own record in office, assuming he wins in the overwhelmingly Democratic district in November. He was pushed hard in the primary, mostly by state Sen. George Bachrach. It was a cold- shower introduction into a world where no one's votes can be taken for granted.
While famous names may stay the same, times change. The Vietnam war, which many think his Uncle Jack helped get the United States into, was later notably repudiated when Congressman O'Neill became the first establishment Democrat to part with Lyndon Johnson on the issue. Young Joe is for capital punishment; he supports President Reagan's bomb-Libya policy. Hardly fundamentalist liberal stands. Yet he has established an image of caring for the needs of the disadvantaged and the elderly -- traditional Kennedy stands.
Joe's sister Kathleen Kennedy Townsend, running for Congress in Maryland, may also represent the political future. Women are gaining in races at all levels. Young ``high-tech'' professionals identified more with Joe's opponents. Interest in the 1986 Kennedy races is not in the family name alone, but in how familiarity and change commingle.