A disputed policy that would require union rules, benefits and oversight on County of Sonoma construction projects of $10 million or more was formally approved Tuesday on a 4-0 vote by the Board of Supervisors.

The decision greenlighting project labor agreements was hailed by union officials as a historic vote that will bolster the middle class. Unaffiliated builders and trade groups, however, lambasted the policy, saying it will undermine competitive bidding and drive up taxpayer construction costs.

The two camps, which have battled over the issue for years, are now bracing for a wider fight as unions prepare to seek similar labor deals across the public sector in Sonoma County.

The next stop is Santa Rosa, where union leaders said the City Council may take up the issue next month in a study session.

"We believe this will be the beginning of using project labor agreements to manage risk on these very complicated public projects," said Jack Buckhorn, president of the North Bay Labor Council, a large coalition of private and public labor groups that has spearheaded the push for the union rules.

The vote was scheduled on the supervisors' consent agenda and came with almost no board discussion and no one from the audience rising to speak. That was in stark contrast to the five-hour hearing two weeks ago, when the board gave its preliminary endorsement after hearing from more than 70 speakers voicing their support or opposition to the policy.

Supervisors Shirlee Zane, Mike McGuire, Susan Gorin and Efren Carrillo voted to approve the measure Tuesday. Supervisor David Rabbitt, the board chairman, was absent from the meeting on family business.

The approval came over the continued objection of non-union builders, who say such deals discourage their firms from competing for projects, force them to act as union employers and drive up construction costs.

The county "will get far less competitive bidding now," said Monica Soiland Nelson, president of the North Coast Builders Exchange, a Santa Rosa-based trade group. She claimed the vote amounted to a show of political loyalty by supervisors to union interests.

"It almost seemed like the supervisors voted on it like they had to and not because they were really truly looking at what the best decision was for taxpayers," Soiland Nelson said.

Union leaders, for their part, have acknowledged their newfound political strength on the county board. Their majority was solidified by Gorin's election in late 2012, months after a differently composed Board of Supervisors opposed a previous project labor policy on a 3-2 stand, without a formal vote. Carrillo and Rabbitt were in the majority, with Zane and McGuire in the minority.

Supporters tout project labor agreements as a way to promote local hiring, enhance job training and extend union benefits to nonunion workers. They reject claims that such deals add to taxpayers' construction costs, citing terms that they say limit cost overruns and labor strife on large construction projects.

"Until you actually have a (project labor agreement) and live it and see it and breathe it, you're going to have some reservations," Buckhorn said. "We're going to prove that it works."

The policy approved by the board was stripped two weeks ago of a key provision opposed by unions but sought by Rabbitt and Carrillo to show any impacts such contracts would have on taxpayer costs. It would have established an alternative bid process without the union rules to compete against bids conforming to union rules.

Labor leaders said it would have created a loophole that would allow non-union contractors to win projects by underbidding and adding costs on the back end. Zane and McGuire -#8212; both of whom enjoy strong union support -#8212; opposed the provision. Gorin, who initially sought a compromise, later joined them in the majority.

Carrillo said Tuesday he was disappointed by the move. "This was a recommendation that would have provided the county more flexibility and balance in its policy," he said.

The policy also caps the number of existing employees a nonunion contractor could bring to a job, requiring the remainder to come through local union hiring halls.

Project labor agreements are common in California, both in the public and private sectors, but as few as three counties in the state have policies requiring such deals on their construction projects.

The county has identified three projects that would qualify under the policy: a $68 million detention and probation facility, a new $50 million airport terminal, and phased administrative center upgrades, each with a potential value greater than $25 million.

Projects of the Sonoma County Water Agency would not be covered by the policy.

Gorin, in the only board comments on the issue, called the number of qualifying projects "narrow in scope."

"We just needed to clarify that it would cover a couple of the projects moving forward but not all of the contracts we routinely approve or have sent out for bid," she said.

Ken Kreischer, chief financial officer of Santa Rosa-based Western Water Constructors Inc., took issue with Gorin's justifying stance.

"If our concerns are valid, then it doesn't matter if there is one project or 100 projects," he said.

Union leaders said they would be seeking a lower dollar threshold for qualifying projects in discussions with Santa Rosa, where they said supporters at this point include council members Julie Combs, Erin Carlstrom and Gary Wysocky.

"I don't know if I have the fourth vote, to be honest," said Buckhorn. "It will be another big fight."