Joseph Kennedy III, the son of former Rep. Joe Kennedy (D-Mass.) and grandson of the late Sen. Robert Kennedy (D-N.Y.), will announce his bid for Congress in the Bay State’s 4th district on Thursday, a campaign source confirmed to Roll Call.

Kennedy announced an exploratory committee last month and just about cleared the Democratic primary field for the open seat, currently held by retiring Rep. Barney Frank (D).

Kennedy will likely face 2010 GOP nominee Sean Bielat in November, but recent polls have shown the scion of the Massachusetts political dynasty with a huge lead.