The Trump administration may be quietly conceding defeat to California on car tailpipe emissions, the biggest battleground in the state’s showdown with President Trump over climate change.

Environmental Protection Agency chief Scott Pruitt backed away last month from his threats to challenge California’s unique legal authority, known as a waiver, to set aggressive limits on vehicle emissions, including greenhouse gases.

Although Pruitt left the door open to a future challenge, experts said he is running out of time to stop California from dictating national pollution standards on cars, the nation’s primary source of greenhouse gas emissions.

“The auto manufacturers aren’t going to make two different kinds of cars, California and non-California, so by default they’re really required to make cars to the California standards,” said Michael Steel, a lawyer in the San Francisco office of the Morrison & Foerster firm who advises companies on environmental compliance.

Because of the long lead time needed to design cars, Steel said, “It’s kind of too late” for the administration to block California’s rules. “There’s a timing issue in terms of whether you can effectively turn the clock back any later than now.”

California is the nation’s largest car market, and a dozen other states, comprising more than 40 percent of the U.S. population, have adopted California’s emissions standards.

Last week’s decisions by Chinese-owned Volvo to put electric engines in all its new cars, and by France to phase out gasoline and diesel cars by 2040, only strengthened California’s hand.

“I don’t want to attribute any one automaker’s statements to our regs,” said Joshua Cunningham, head of the California Air Resources Board’s clean cars branch, which develops the state’s car pollution standards. “But given the broad momentum of California’s regulations and what’s happening in Europe and in China, I think the industry sees some pretty consistent signals from a lot of governments that long-term emissions requirements are going to continue to get more strict.”

Pruitt had refused in his confirmation hearing to guarantee that California could continue to set its own tailpipe standards, a right that was explicitly written into the federal Clean Air Act five decades ago to help the state combat smog in the Los Angeles Basin.

The state is using that authority to meet its aggressive climate goals by squeezing carbon emissions from gasoline engines and beginning a paradigm shift toward electric cars and other zero-emission vehicles.

The Obama administration, also trying to tackle climate change, had worked with the state to toughen federal tailpipe standards along the same lines.

Just before President Barack Obama left office, the EPA rushed out a decision, called a midterm review — a dense set of technical findings years in the making. It said the tougher emissions standards are feasible, clearing the way to impose them nationwide.

Detroit automakers rebelled, saying the new rules were too onerous, and Pruitt moved quickly to revoke the midterm review. The agency has until next spring to come up with a new determination, but any changes would require the EPA to reverse its technical findings and be able to defend that reversal in court.

But Pruitt held off on attacking California’s waiver, and state officials warned that they would fight him in court should he try. The waiver is legally ironclad, lawyers said, and has bipartisan political support in the state.

Last month, in response to a question from House Appropriations subcommittee chair Ken Calvert, a Riverside County Republican who supports the waiver, Pruitt said, “Currently, the waiver is not under review,” while praising the state’s “leadership” on air pollution.

Mike Dovorany of the CarLab, a consulting firm that helps carmakers and suppliers plan new vehicles, said while carmakers may not like California’s standards, they cannot risk failing to meet them. As long as the waiver remains, carmakers “have to plan those vehicles,” Dovorany said. “There’s no way around it.”

As it is, he said carmakers are struggling to comply. The state standards will require at least 8 percent of new California car sales to be plug-in or otherwise zero emission by 2025, more than four times their current share. The way the rules work, he said, manufacturers that fail to comply would be banned from selling vehicles in California, a disaster for any carmaker.

Meeting the new standard will be “extremely tough,” Dovorany said. “That’s why any coasting, if you will, on trying to plan for that compliance is just such a dangerous game for any car manufacturer.”

Even the Alliance of Automobile Manufacturers, which urged Pruitt to revisit the federal rules, said it is not pushing to revoke California’s waiver.

“We’ve not asked for the agency to review that decision,” the group said when asked for its position.

Peter Hsiao, head of the environment and energy attorney group in the Los Angeles office of Morrison & Foerster, said Pruitt has probably been advised by EPA lawyers that “there is no legal basis” to revoke the waiver. “I think they’re moving on to other issues,” Hsiao said. “This is one they looked at and decided not to take on.”

Other observers caution against reading too much into Pruitt’s assurances. Not only did he leave open the possibility of future action, but allowing California to plunge ahead with tougher tailpipe rules would defeat the purpose of the administration’s attempt to roll back the federal rule.

“If they really think the standards need to be loosened, why would they let 40 percent of the country continue to be under the California standards, which California could even strengthen?” said Ann Carlson, a professor of environmental law at UCLA.

California officials said they are moving ahead with their new rules.

“Mr. Pruitt has other priorities at this point, and we’re fine with that,” said Dave Clegern, spokesman for the California Air Resources Board.

“This is the way the world is going,” Clegern added. “It doesn’t matter if the U.S. is going that way or not. It’s a huge market that’s going to be developing, and we think that all the automakers see that, and they are all doing their best to get ready for it.”

China has displaced the United States as the world’s largest market for passenger vehicles.

Volvo’s decision “reinforces trends toward greater efficiency and eventual decarbonization of the transportation sector,” said Drew Kodjak, head of International Council on Clean Transportation, a nonprofit research group. “China is becoming more and more aggressive in entering the global stage,” he said, calling Volvo’s move a big signal that “China is going to make a play for leapfrogging over the internal combustion engine.”

Carlson warned that Pruitt’s efforts to help Detroit by weakening federal standards will backfire.

“The Trump administration is essentially ceding the green economy to other countries,” Carlson said. “Trump really is investing in very old-school notions that efficiency doesn’t matter, that renewable fuels are not the way to go. There are really important economic repercussions for failing to address climate change, not just environmental repercussions.”

Carolyn Lochhead is The San Francisco Chronicle’s Washington correspondent. Email: clochhead@sfchronicle.com Twitter: @carolynlochhead