SACRAMENTO — When a company called Ausra filed plans for a big solar power plant in California, it was deluged with demands from a union group that it study the effect on creatures like the short-nosed kangaroo rat and the ferruginous hawk.

By contrast, when a competitor, BrightSource Energy, filed plans for an even bigger solar plant that would affect the imperiled desert tortoise, the same union group, California Unions for Reliable Energy, raised no complaint. Instead, it urged regulators to approve the project as quickly as possible.

One big difference between the projects? Ausra had rejected demands that it use only union workers to build its solar farm, while BrightSource pledged to hire labor-friendly contractors.

As California moves to license dozens of huge solar power plants to meet the state’s renewable energy goals, some developers contend they are being pressured to sign agreements pledging to use union labor. If they refuse, they say, they can count on the union group to demand costly environmental studies and deliver hostile testimony at public hearings.

If they commit at the outset to use union labor, they say, the environmental objections never materialize.

“This does stress the limits of credibility to some extent,” a California energy commissioner, Jeffrey Byron, said at one contentious hearing, “when an attorney representing a labor union is so focused on the potential impact of a solar power plant on birds.”

Union leaders acknowledge that they make aggressive use of the environmental laws, but say they do it out of genuine concern for the sustainability of California’s power industry, not just as a negotiating tactic. And they contend they do not abandon valid environmental objections to a project just because a company signs a labor agreement.

“We’ve been tarred and feathered more than once on this issue,” said Marc Joseph, a lawyer for California Unions for Reliable Energy. “We don’t walk away from environmental issues.”

At proposed fossil-fuel power plants, the union group has long been accused of exploiting environmental laws to force companies into signing labor agreements. The tactic is a subject of perennial discussion in the California legislature, which has considered, but never passed, bills to strip labor of its right to participate in environmental assessments.

What is new is that California Unions for Reliable Energy, a coalition of construction unions, appears to be applying this approach to new-age renewable energy projects, especially solar power plants, which are being fast-tracked to help meet the state’s green power target.

Lawyers for the union both negotiate labor agreements with solar developers and participate in the environmental review of the projects.

California Unions for Reliable Energy insists it is pursuing the long-term interests of its members. If energy projects are held to high environmental standards, the group says, more of them will ultimately get built, and that will mean more union jobs.

Nationwide, as billions of dollars in public and private investment flow to renewable energy projects, the environmental and labor battles being fought in California could prove to be the opening skirmishes of a larger fight over the emerging green economy.

Should Rust Belt factories converted to making solar components and wind turbines be union shops, gateways to the middle-class for a new generation of workers in the green economy? Or will the green economy look more like the service economy, with low-paid employees installing rooftop solar panels and retrofitting buildings?

For the labor movement, green jobs represent an opportunity to regain relevance after years of declining membership.

“Unions are trying to get a foothold in solar, wind and other new green occupations,” said Philip Mattera, research director for Good Jobs First, a labor-oriented research group in Washington.

“We’re at a turning point that will have an impact on the future of the whole economy, and a lot of unions are gearing up.”

But skeptics fear that union control of renewable energy projects will saddle the nascent industry with high costs and undermine its competitiveness.

“These environmental challenges are the unions’ major tactic to maintain their share of industrial construction — we call it greenmail,” said Kevin Dayton, state government affairs director for the Associated Builders and Contractors of California. “The future of solar energy is jeopardized by these unions holding up construction.”

In California, project labor agreements can raise costs on a project by about 20 percent, Mr. Dayton estimated.

The fights of the moment center on solar farms proposed for tens of thousands of acres of desert and agricultural land.

When the utility giant the FPL Group ignored entreaties from California Unions for Reliable Energy to use union labor on a planned 250-megawatt solar farm, it was hit with 144 data requests, demanding details on things like “the engine brand, model, and horsepower rating” of a water pump engine, “the number of man-hours devoted to focused tortoise surveys, by location” and “the role of each individual that participated.”

In filings with the California Energy Commission, Ausra has accused the union group of abusing environmental laws in a bid to extract a labor agreement. FPL’s lawyers accused the group of trying to stall the company’s solar project.

Bob Balgenorth, chairman of the labor group, makes no apologies for pushing hard for union jobs from solar developers while scrutinizing the environmental impact of the projects. “You only have so much land that can accept solar power plants,” said Mr. Balgenorth, who has cultivated strong ties with conservation groups.

“So the question is, should that land be used for low-paid jobs or should that land be used for high-paid jobs?”

Some solar developers say that signing a labor agreement is simply an unavoidable cost of doing business.

“Let’s just say that it is clear to us from experience that if we do not enter into a project labor agreement, the costs and schedule of the project is interminable,” said Douglas Wert, chief executive of Spinnaker Energy, a San Diego company hired to build two solar farms for Portuguese developer Martifer.

After Stirling Energy Systems filed plans with California regulators to install 30,000 solar dishes on 10 square miles of desert land, its executives got a call from Mr. Joseph, the union lawyer. Sean Gallagher, a vice president for Tessera Solar, the development arm for Stirling, said the company declined Mr. Joseph’s request to commit to using union labor.

California Unions for Reliable Energy subsequently filed 143 data requests with the company on the final day such requests could be made, and later intervened in a second, 850-megawatt Stirling solar project.

It was a different story after BrightSource Energy pledged to hire union-friendly contractors to build its Mojave Desert solar power plant complex. Despite questions raised by environmental groups about the project’s impact on wildlife, the union group took no action, according to commission documents.

Mr. Joseph said that the labor group wants to mediate between environmentalists and BrightSource, which is based in Oakland, Calif.

“We’re actually hoping that we can help resolve these issues in a way that allows that project to go forward and gives maximum protection to the desert tortoise,” he said.

He said he sees “absolutely no conflict of interest” in seeking labor agreements from solar developers while challenging the environmental effect of the projects. “It is in the interest of construction workers to have good middle class jobs — and to have conventional and renewable power plants that are sustainable,” Mr. Joseph said.

The union group’s strategy drew plaudits from environmentalists when the group was winning agreements from developers to cut pollution from fossil fuel power plants. But as some conservation groups ally themselves with business interests to push for a rapid rollout of renewable energy, strains are showing in the so-called blue-green alliance.

Some environmental groups are worried that the labor tactics will delay green energy projects and cause a backlash, but they are reluctant to go public with criticisms of the labor movement.

Others, like the Natural Resources Defense Council, are trying to steer clear of the controversy.

The council “hasn’t taken a position on whether union labor should or shouldn’t be used in these projects,” said Sheryl Carter, the group’s co-director of energy programs.

And still others defend the labor group’s role.

Carl Zichella, the Sierra Club’s director of western renewable programs, said California Unions for Reliable Energy had been effective at extracting concessions that aid the environment.

“It’s not a warm fuzzy thing they are doing; it’s a very self-interested thing they’re doing,” he said. “But it has a large ancillary public benefit.”