The California Nurses Association, the 85,000-strong union that flexed its muscle last election season with relentless attacks on Republican gubernatorial candidate Meg Whitman, dived into the San Francisco mayor’s race Tuesday by endorsing Leland Yee, the state senator representing the western part of San Francisco and San Mateo County. 

The early endorsement from one of the state’s most formidable political organizations is a boost for a candidate already widely considered to be a front runner in the crowded mayor’s race. Last year, the CNA’s no-holds-barred brand of electoral warfare was on full display during the governor's race. The union staged some of the season’s most damaging publicity stunts, from protests outside Whitman's Atherton home featuring an actress dressed as "Queen Meg" to the undocumented-maid scandal that the union reportedly helped orchestrate. 

The CNA nod is merely the latest in a growing stack of union endorsements for Yee. The American Federation of State, County and Municipal Employees announced its support for him last month, one week after he secured the backing of the local building trades union. 

But it is not fully clear how the significant support from statewide and national labor groups will translate into an advantage at the city level. The San Francisco Labor Council, the most influential labor endorsement locally, is not expected to tip its hand for several months. It is also clear how much money the nurses’ union will spend in this local contest — or in what way it will battle on Yee’s behalf.

Bonnie Castillo, the governmental relations director for the union, said there is no current plan to launch an independent campaign on Yee’s behalf.

“We’ll evaluate that,” she said.

With San Francisco’s mayoral field packed with liberal Democrats, the union is unlikely to unleash its firepower as it did last fall against Whitman and Sharron Angle, the Tea Party-backed challenger to Senate Majority Leader Harry Reid, said Chris Lehane, a Democratic opposition researcher who has worked with the CNA in the past.

Given the importance of a candidate’s favorability in the public eye under the new ranked-choice voting system, Castillo added, Yee supporters will likely avoid overt attacks.

“It’s not something that we like to do,” Castillo said. “Everyone’s looking at ranked voting in a thoughtful light, and I think our committee of rank-and-file nurses are really wanting on focus on the more positive aspects of the candidate.”