Boston City Councilman Mike Ross (D) will not run for Massachusetts’ 4th district open seat, he announced Monday.

Ross was the first potential candidate to form an exploratory committee to succeed retiring Rep. Barney Frank (D). His decision to forgo a bid practically clears the field for attorney Joseph Kennedy III, who has formed an exploratory committee and is likely to announce a full-blown campaign soon.

Kennedy, the son of former Rep. Joe Kennedy, will have all the benefits — and burdens — of being the scion of America’s most storied political family.

While two Republicans are vying for their party’s nomination, Democrats are almost certain to hold the district. Roll Call rates the race Safe Democratic.