BURLINGTON - Though a number of others expressed interest initially, the race for the vacant state Senate seat in the Fourth Middlesex District has narrowed to six - four Democrats, one Republican, and a candidate for the Constitution Party.

On the Democratic side, state Rep. Charles Murphy of Burlington, state Rep. Patrick Natale of Woburn, state Rep. James Marzilli of Arlington, and Kenneth Donnelly of Arlington, secretary-treasurer of the Professional Fire Fighters of Massachusetts, are vying for the nomination.

The only remaining Republican is Brion Cangiamila of Billerica.

Thomas Fallon of Burlington will be on the ballot as the Constitution Party candidate.

Robert Peters, a Democrat, took out nomination papers in the race, but did not return them. Peters is a Lexington Town Meeting member and served as a Town Meeting member in Burlington when he lived in that community.

Linda Fosburg, a Lexington Republican, also did not return nomination papers.

The primary for the special election will be held on Tuesday, Nov. 13. The final election will take place on Tuesday, Dec. 11.

The seat became vacant when Robert Havern, an Arlington Democrat resigned in late August to work for a consulting company associated with a Boston law firm.