The field for retiring Rep. Bill Delahunt's (D-Mass.) seat is filling out fast.

Former state Treasurer Joe Malone officially announced his candidate on Sunday. He joins a primary field that includes state Rep. Jeff Perry.

On the Democratic side, state Sen. Robert O'Leary is in and Norfolk District Attorney William Keating is considering a bid.

Malone served as state treasurer in the 1990s before losing campaign for governor in 1998. He lost the GOP nomination that year to acting Gov. Paul Celucci.

Republicans have made Delahunt's seat a top target ever since he announced his retirement earlier this month. it was their best district in the January special Senate election won by Scott Brown.