Joseph P. Kennedy III, grandson of Robert Kennedy, will leave a Massachusetts prosecutor’s office to pursue a bid for the congressional seat vacated by the retirement of Rep. Barney Frank, D-Mass., sources close to Kennedy told National Journal.

Kennedy, 31, would enter the race with the unmatchable family name and tenuous ties to the district, meeting a still-nebulous field taking shape slowly following Frank’s November announcement that he would not seek re-election this year.

Kennedy will form an exploratory committee around the 4th Congressional district seat, sources said.