The precise contours of this battle will become clear in the months ahead, as Mr. Trump’s environmental policies take shape. For now, the critical questions are whether the United States will withdraw from the Paris agreement, an international compact to reduce greenhouse pollution, and whether the Environmental Protection Agency will revoke a waiver issued by President Richard M. Nixon that permits California to set fuel economy standards exceeding federal requirements.

Revoking the waiver, which was central to a policy that has resulted in noticeably cleaner air in places like Los Angeles, would force the state to lower its tough fuel economy standards, which are also intended to promote the rapid spread of electric cars. As they stand, the rules would force automakers to build fleets of cars that would reach 54.5 miles per gallon by 2025.

California is preparing for a legal challenge. “You have to be concerned when anybody talks about going backward,” said Xavier Becerra, the state attorney general. “In this case we think we have a strong case to be made based on the facts and the history.”

Mr. Trump is already moving to weaken federal auto emission standards that were influenced by California’s tougher standards. Automakers, who met with the president in the Oval Office days after he assumed the presidency, have long complained that the standards forced them to build expensive electric vehicles that consumers may not want.

And the companies have lobbied for years to stop the federal government from allowing California to set cleaner tailpipe regulations than the rest of the nation, arguing that the double standard necessitates building two types of cars. In Detroit, those companies see President Trump as their best chance for finally ending onerous California car requirements. But in the meantime, over a dozen other states have adopted California’s auto emissions standards — and Mr. Brown is betting that the sheer size of that market will be enough to make the Trump administration reconsider any effort to roll back the California waiver.

“Because we’re such a big part of the car market, and places like New York and Massachusetts are tied in with the U.S., our standard will prevail,” he said.

Beyond pushing to maintain its state climate laws, California has tried to forge international climate pacts. In particular, Mr. Brown’s government helped draft and gather signatures for a memorandum of understanding whose signers, including heads of state and mayors from around the world, pledged to take actions to lower emissions enough to keep global temperatures from rising over two degrees Celsius. That is the point at which scientists say the planet will tip into a future of irreversible rising seas and melting ice sheets.