BOSTON — A top Massachusetts Democrat said Saturday that one of Robert F. Kennedy’s grandsons is considering carrying on the family’s vaunted political tradition by running for the U.S. House of Representatives. Joseph P. Kennedy III, one of the twin sons of former Rep. Joseph P. Kennedy II, may run this fall if Democratic Rep. William Delahunt decides against seeking re-election in his South Shore and Cape Cod district. Kennedy, 29, “has been considering it but he hasn’t made a decision,” said the Democrat, who demanded anonymity to speak about private conversations with the father and son.