All four Democratic Senate candidates are now advertising on television.

Alan Khazei joined the fray today with a commercial focusing on his work creating the youth service program City Year and enacting the Serve America Act.

Attorney General Martha Coakley also started advertising this week, following campaigns that have been airing for the past month by U.S. Rep. Michael Capuano and Boston Celtics [team stats] co-owner Stephen Pagliuca.

Khazei has also drafted a six-point plan he says will create 100,000 new jobs.

The candidates are running to replace Sen. Edward Kennedy, who died of brain cancer on Aug. 25.

Republicans Scott Brown and Jack E. Robinson are also seeking the seat.

The primary is Dec. 8; the general election is Jan. 19.