LOS ANGELES — The environmental ministers of Canada and Mexico went to San Francisco last month to sign a global pact — drafted largely by California — to lower planet-warming greenhouse pollution. Gov. Jerry Brown flies to China next month to meet with climate leaders there on a campaign to curb global warming. And a battery of state lawyers is preparing to battle any attempt by Washington to weaken California’s automobile pollution emission standards.

As President Trump moves to reverse the Obama administration’s policies on climate change, California is emerging as the nation’s de facto negotiator with the world on the environment. The state is pushing back on everything from White House efforts to roll back pollution rules on tailpipes and smokestacks, to plans to withdraw or weaken the United States’ commitments under the Paris climate change accord.

In the process, California is not only fighting to protect its legacy of sweeping environmental protection, but also holding itself out as a model to other states — and to nations — on how to fight climate change.

“I want to do everything we can to keep America on track, keep the world on track, and lead in all the ways California has,” said Mr. Brown, who has embraced this fight as he enters what is likely to be the final stretch of a 40-year career in California government. “We’re looking to do everything we can to advance our program, regardless of whatever happens in Washington.”

Since the election, California has stood as the leading edge of the Democratic resistance to the Trump administration, on a range of issues including immigration and health care. Mr. Trump lost to Hillary Clinton here by nearly four million votes. Every statewide elected official is a Democrat, and the party controls both houses of the Legislature by a two-thirds margin. Soon after Mr. Trump was elected, Democratic legislative leaders hired Eric H. Holder Jr., the former attorney general, to represent California in legal fights with the administration.

But of all the battles it is waging with Washington, none have the global implications of the one over climate change.

The aggressive posture on the environment has set the stage for a confrontation between the Trump administration and the largest state in the nation. California has 39 million people, making it more populous than Canada and many other countries. And with an annual economic output of $2.4 trillion, the state is an economic powerhouse and has the sixth-largest economy in the world.

California’s efforts cross party lines. Arnold Schwarzenegger, who served as governor from 2003 to 2011, and led the state in developing the most aggressive pollution-control programs in the nation, has emerged as one of Mr. Trump’s biggest Republican critics.

Mr. Trump and his advisers appear ready for the fight.

Scott Pruitt, the Environmental Protection Agency chief, whom Mr. Trump has charged with rolling back Obama-era environmental policies, speaks often of his belief in the importance of federalism and states’ rights, describing Mr. Trump’s proposals as a way to lift the oppressive yoke of federal regulations and return authority to the states. But of Mr. Brown’s push to expand California’s environmental policies to the country and the world, Mr. Pruitt said, “That’s not federalism — that’s a political agenda hiding behind federalism.”

“Is it federalism to impose your policy on other states?” Mr. Pruitt asked in a recent interview in his office. “It seems to me that Mr. Brown is being the aggressor here,” he said. “But we expect the law will show this.”

In one of his earliest strikes, Mr. Trump signed an executive order aimed at dismantling the Clean Power Plan, President Barack Obama’s signature climate policy change. Much of the plan, which Mr. Trump denounced as a “job killer,” was drawn from environmental policies pioneered in California.

Mr. Brown has long been an environmental advocate, including when he first served as governor in the 1970s. He has made this a central focus as he enters his final 18 months in office. In an interview, he said the president’s action was “a colossal mistake and defies science.”

“Erasing climate change may take place in Donald Trump’s mind, but nowhere else,” Mr. Brown said.

The leadership role embraced by California goes to the heart of what has long been a central part of its identity. For more than three decades, California has been at the vanguard of environmental policy, passing ambitious, first-in-the-nation measures on pollution control and conservation that have often served as models for national and even international environmental law.

“With Trump indicating that he will withdraw from climate change leadership, the rest of the global community is looking to California, as one of the world’s largest economies, to take the lead,” said Mario Molina, a Nobel Prize-winning scientist from Mexico who advises nations on climate change policy. “California demonstrates to the world that you can have a strong climate policy without hurting your economy.”

The Senate leader, Kevin de Leon, introduced legislation this month that would accelerate, rather than retrench, California’s drive to reduce emissions, requiring that 100 percent of retail electricity in the state come from renewable sources by 2045. Mr. de Leon said it was “important that we send a signal to the rest of the world” at a time of what he described as “blowback” from Washington.

Mr. Schwarzenegger, who tangled with Mr. Trump after the president mocked him for receiving low ratings as his replacement on “The Apprentice,” described Mr. Trump’s environmental policies as a threat to the planet.

“Saying you’ll bring coal plants back is the past,” Mr. Schwarzenegger said. “It’s like saying you’ll bring Blockbuster back, which is the past. Horses and buggies, which is the past. Pagers back, which is the past.”

He said California had shown it could adopt aggressive environmental policies without hurting the economy. “We’re outdoing the rest of the country on G.D.P.,” Mr. Schwarzenegger said.

Even before Mr. Trump took office, California’s tough regulatory rules had stirred concern among business leaders, who said it had increased their costs. They warned that the situation would become worse if California stood by its regulatory rules while Washington moved in the other direction.

“We’re very concerned about that,” said Robert C. Lapsley, the president of the California Business Roundtable. “If we are 1 percent of the problem, and we have the most far-reaching climate policies on the planet while all the other states are slowing down because Washington is slowing down, that is going to create an absolute imbalance.”

“Washington will create a less competitive environment for California businesses here because businesses in other states will not have to meet the same mandates,” he added. “There is no question that businesses are going to move out.”

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.

That pact is voluntary, but California, Canada and Mexico are starting to carry out a joint policy with some teeth.

California’s signature climate change law is the cap-and-trade program. It places a statewide cap on planet-warming carbon dioxide emissions, and then allows companies to buy and sell pollution credits. The California measure was the model for a national climate law that Mr. Obama tried unsuccessfully to have passed in 2010.

Given the setbacks in Washington, California environmental officials are working with Mexico and Canada to create what is informally called the “Nafta” of climate change — a carbon-cutting program that spans the region.

“Canada’s all in when it comes to climate action, and we’ll partner with anyone who wants to move forward,” said Catherine McKenna, Canada’s environment minister.

Already, California’s cap-and-trade market is connected to a similar one in Quebec, now valued at about $8 billion, and the Province of Ontario is linking with the joint California-Quebec market this year. Climate policy experts in Sacramento and Mexico City are in the early stages of drafting a plan to link Mexico with that joint market.

In April, a delegation from California traveled to Beijing to meet with Chinese counterparts to help them craft a cap-and-trade plan. “We have people working in China, in their regulatory agencies, consulting with them, speaking fluent Mandarin, working with the Chinese government — giving them advice on cap and trade,” Mr. Brown said.

The Clean Power Plan was central to the United States’ pledge under the 2015 Paris agreement, which commits the nation to cut its emissions about 26 percent from 2005 levels by 2025. Now that Mr. Trump has moved to roll back the plan, it will be almost impossible for the United States to meet its Paris commitments.

That has resonated powerfully in China. The heart of the Paris agreement was a 2014 deal forged by Mr. Obama and President Xi Jinping of China in which the world’s two largest economies and largest greenhouse polluters agreed to act jointly to reduce their emissions.

“China is committed to establishing a cap-and-trade this year, and we are looking for expertise across the world as we design our program — and we are looking closely at the California experience,” said Dongquan He, a vice president of Energy Foundation China, an organization that works with the Chinese government on climate change issues.

Mr. Brown recently met with the prime minister of Fiji, who will serve as chairman of this fall’s United Nations climate change meeting in Bonn, Germany, which aims to put the Paris agreement in force, with or without the United States. The governor said he planned to attend as a representative of his state.

“We may not represent Washington, but we will represent the wide swath of American people who will keep the faith on this,” he said.