Joseph P. Kennedy III, a grandson of former attorney general Robert Kennedy, formally kicked off his candidacy for Congress on Thursday, stopping at the Newton Center subway stop. Kennedy, an assistant district attorney, formally announced his campaign in a video message released Thursday morning, and then he took to an MBTA stop at a Boston suburb in his district. Saying he is “very, very proud of my family’s public record,” Kennedy insisted this campaign is not about his family name but about the issues facing Massachusetts. A recent poll showed Kennedy had a wide lead over his possible opponent, Republican Sean Bielat. Kennedy will hold a fundraiser Wednesday on what would have been his great-uncle Edward Kennedy’s 80th birthday.