Filner's backing came from the Municipal Employees Association, which represents the city of San Diego's white collar workers. The organization has sat out the last few citywide elections.
"He's been an advocate for working and middle class San Diegans in Congress for many years,'' said Bob Cronk, the MEA's political action chairman. "Congressman Filner has nothing to prove and isn't swayed by the political rhetoric of the day.''
MEA President Tony Ruiz said there is "no better candidate to lead our city back to greatness.''
It was the second union endorsement for Filner within a week, following an announcement of support from the San Diego and Imperial Counties Labor Council. The International Brotherhood of Electrical Workers Local 569 and Ironworkers Local 229 are also supporting Filner, who represents San Diego's South Bay neighborhoods.
As the only major Democrat running in the nonpartisan race, he has also been endorsed by the San Diego County Democratic Party and San Diego Democrats for Equality, formerly known as the San Diego Democratic Club.
For Fletcher, support came from the Neighborhood Market Association, which represents 2,000 mostly family-owned retailers on the West Coast.
Fletcher would be a strong advocate for small business as mayor, said Mark Arabo, president and chief executive officer of the association.
"Nathan understands that excessive taxes, fees and regulations are strangling small businesses in San Diego,'' Arabo said. "Nathan's proven track record of building coalitions to get things done is all the evidence we need.''
The NMA is the fourth business group to endorse Fletcher.
He and Filner will face Councilman Carl DeMaio and District Attorney Bonnie Dumanis, among others, in the June 2012 primary.