John J. Dougherty, whose mayoral ambitions made him one of the most visible Democrats in Philadelphia politics over the last few years, said yesterday that he would not be running for mayor after all.

At least, not this year.

Dougherty, 46, the leader of the powerful electricians' union and former treasurer of the city's Democratic Party, cited family health reasons. His wife and mother have been ill.

Barring any surprises, his decision appears to finalize the field of candidates for the May 15 Democratic mayoral primary: U.S. Reps. Bob Brady and Chaka Fattah, State Rep. Dwight Evans, former City Councilman Michael Nutter, and businessman Tom Knox.

With former City Controller Jonathan Saidel having also opted out, the field now includes three African American candidates and two whites. Traditionally, Philadelphia voters have broken along racial lines, meaning that candidates benefit from facing the smallest possible number of candidates of their own race.

Dougherty's decision could thus be a boon to Knox and Brady, who are white. Brady, who declined comment yesterday, saw Dougherty as a competitor in his base of support among blue-collar white voters.

The idea that the man known as "Johnny Doc" would be helping Brady - even inadvertently - represents a significant irony. Brady, who runs the city Democratic Party, had a nasty falling-out with the union leader last spring and engineered Dougherty's ouster from his post as party treasurer.

After the split, Dougherty lowered a high profile that had, up to then, featured airborne banners advertising his mayoral hopes to Jersey Shore sunbathers, numerous "Draft John Dougherty '07" fund-raisers, and a dissemination of "Doc '07" caps and T-shirts that began soon after the 2003 election. Also facing Dougherty was an FBI tax-fraud investigation into a local electrical contractor who is his close friend. In November, a search warrant was executed at Dougherty's South Philadelphia house as part of the probe. Dougherty has said repeatedly that he did nothing wrong.

Asserting that he had long been subjected to the "higher standard" of scrutiny as a labor leader, Dougherty said in an interview that the FBI probe had nothing to do with his exit.

"I have never allowed that to paralyze my ability to do what I have to do," he said.

Dougherty is still likely to be a force in the campaign. His union has traditionally been a major player in city politics by virtue of its large donations to candidates such as Mayor Street and Gov. Rendell. Members of Dougherty's Local 98 of the International Brotherhood of Electrical Workers have also acted as vocal - and sometimes controversial - foot soldiers in recent campaigns.

Local 98's federal political fund had more than $2.1 million in cash as of Dec. 31. Separately, DJD '07, the state committee created to support Dougherty's mayoral efforts, reported $64,000 in its last filing, in May.

Perhaps mindful of that, several of the mayoral candidates reached out to Dougherty yesterday. The first to get through, Dougherty spokesman Frank Keel said, was Nutter.

Dougherty said he had made no endorsement decision. But nearly any choice would involve some hatchet-burying.

Over the years, he has bitterly criticized Nutter, who clashed with Local 98 as chairman of the board of the Pennsylvania Convention Center. Likewise, the union has drawn complaints alleging discriminatory hiring practices from groups including the African American Chamber of Commerce, whose founder, A. Bruce Crawley, is a prominent Evans backer.

And last year's feud shows that there was no love lost between Dougherty and Brady - though the latter is famed for his ability to broker truces between warring Democratic factions. A Brady campaign source said that the two had spoken recently but that the campaign was surprised by Dougherty's exit.

By contrast, Dougherty raised eyebrows last week in political circles when he helped underwrite a Fattah-led effort that offered people $200 grocery vouchers in exchange for turning in guns. Fattah had touted the initiative in a speech on crime several days earlier.

Dougherty made it clear that he still has his eyes on a run in 2011 or beyond. "You get one mom, and I get many elections," he said.

Contact staff writer Michael Currie Schaffer at 215-854-4565 or mcschaffer@phillynews.com.

Inquirer staff writer Marcia Gelbart contributed to this article.