By Daniel Lovering

NEWTON, Massachusetts (Reuters) - Joseph Kennedy III easily won the Massachusetts congressional seat being vacated by the retiring fellow Democrat Barney Frank, becoming the first member of his generation of a storied U.S. political family to hold public office.

Kennedy, grandson of the late Senator Robert F. Kennedy, was virtually assured of victory over Republican Sean Bielat after he defeated two lesser-known Democratic Party rivals in the September primary. The district is solidly Democratic and last elected a Republican in 1944.

As supporters and family members gathered at a hotel in the Boston suburb of Newton, Kennedy's campaign reported late Tuesday he had captured 62 percent of the vote versus Bielat's 36 percent, with 80 percent of precincts reporting.

Kennedy, 31, is heir to one of America's most storied political dynasties, which also produced President John F. Kennedy and Senator Edward M. Kennedy.

He will fill the House seat being vacated next year by veteran Democratic Congressman Frank, who formerly served as chairman of the House Financial Services Committee.

(Reporting By Daniel Lovering, editing by Scott Malone, Bernard Orr)