The race to succeed retiring Rep. Bill Delahunt (D) looks likely to come down to two Democrats and two Republicans.

State Sen. Robert O’Leary (D) said Monday that he is definitely in the race and plans to set up a campaign committee this week.

O’Leary’s primary competition for the seat is likely to be district attorney William Keating, whose spokesman told local press outlets to expect an announcement in the coming days.

Keating does not live in the district but has a base of support in the urban and suburban areas around Quincy and Weymouth. O’Leary is from Barnstable and counts the district’s more rural Southern Cape and islands as his base.

Among other Democrats, State House Assistant Majority Leader Ronald Mariano ruled out a run Monday. State Rep. James Murphy and Ian Bowles, secretary of the Executive Office of Energy and Environmental Affairs, did not respond to inquiries.

“I think it’s going to be a very competitive primary,” O’Leary said.

The winner of the Democrats’ Sept. 14 primary will face the winner of the GOP primary, which includes two top-tier candidates: state Rep. Jeffrey Perry and former state Treasurer Joe Malone.

Malone intends to formally launch his candidacy on March 21.