The Trump administration has released revamped rules that would unravel strict Obama-era standards for tailpipe emissions and gas mileage in new cars and trucks and directly challenge California’s authority to determine its own emissions limits.

The long-anticipated proposal offers a range of scenarios but singles out a preferred alternative: freezing fuel-efficiency standards in 2020 at an average 35 miles per gallon. It will almost certainly face a barrage of legal challenges, but any rollback would be a blow to California’s ambitions for limiting planet-warming greenhouse gases.

Reducing transportation emissions is a critical component of the state’s approach to climate change, a defining policy for California. Transportation accounts for nearly 40 percent of the state’s harmful air pollution, and existing state rules directly reduce tailpipe emissions, becoming more stringent over time.

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As a practical matter, however, the proposed changes could languish in courts for years, allowing California to stay its course in the near term. State officials have vowed take aggressive legal action to prevent implementation of the new standards. California, along with dozens of other states, sued the administration in May, challenging the administration’s right to revisit the existing rules.

Gov. Jerry Brown, in a written statement Thursday, termed the proposed rollback a “reckless scheme” that would mean “motorists will pay more at the pump, get worse gas mileage and breathe dirtier air. California will fight this stupidity in every conceivable way possible.”

The rules set at the end of the Obama administration require an average 45.4 miles per gallon by 2022 and more than 50 miles per gallon by 2025. Standards differ by vehicle type and are more stringent for cars than for SUVs and light trucks.

Trump administration officials said the revised rules would make cars more affordable and safer for consumers. “More realistic standards will promote a healthy economy by bringing newer, safer, cleaner and more fuel-efficient vehicles to U.S. roads,” Transportation Secretary Elaine L. Chao said in a statement.

The proposal, announced by the Environmental Protection Agency and the National Highway Traffic Safety Administration, are open to negotiation, and at least one federal official left the door open for compromise. EPA assistant administrator Bill Wehrum said, “We are committed to working with California to try to find a mutually agreeable set of regulations,” according to the Los Angeles Times.

The proposal will undergo 60 days of public comment before being finalized.

Of the many dozen skirmishes between Washington and Sacramento, the emissions revisions are among the most contentious, touching on one of the state’s most intractable problems: air pollution.

Mary Nichols, who chairs the state Air Resources Board, echoed Brown’s defiant tone.

“Stay tuned for further comment,” she said in a statement. “Meantime, California remains fully committed to a rigorous 50-state program with a full range of vehicle choices. That program is in effect right now and will remain so for the foreseeable future.”

Perhaps most troubling for state officials, the proposed rules take aim at California’s longstanding autonomy in protecting its air quality and public health. Congress, under the Clean Air Act, granted the state a unique right to forgo federal emissions standards and chart its own regulatory course.

That waiver has been a critical tool for California to scrub much of the smog from the L.A. basin and leverage for the state’s push to reduce transportation emissions. Losing the right to self-regulation takes away the consistency state regulators want to achieve and puts California at the mercy of each administration that could revisit the rules as it wishes.

More than a dozen other states have adopted California’s standards. Environmental groups and health organizations, such as the American Lung Association, say the proposed rules would return congested cities to the smoggy, unhealthful traps they once were.

“It would be a violation of our public health and air quality nationally,” Bill Magavern, a lawyer and policy director for the advocacy group Coalition for Clean Air. Most pollution in California “is coming from vehicles, so (existing) rules are essential to public health.”

Agencies and activists are still evaluating the implications of the proposal. But some legal experts say the entire package will be vigorously challenged in court.

“The federal government cannot take away permission that its already granted,” said Irene Gutierrez, an attorney with the environmental group Natural Resources Defense Council. “It would be completely contrary to how Congress set up the program for California, which was about the need to have its own regulatory powers to protect citizens of the state.”

Sean Donahue is a lawyer representing the Environmental Defense Fund, which, along with other environmental groups, sued the EPA in May, after then-chief administrator Scott Pruitt announced that Washington would revisit the emissions rules. Donahue called the EPA’s move “an extraordinary attack on California’s historic role….There’s going to be a huge fight.”

One argument he and others will make is that the federal agency has never revoked any of the more than 100 waivers it has granted to California since the 1960s. A court might consider such a pivot a “violent rupture with precedent,” Donahue said.

Another strategy, some experts say, is to question the administration’s rationale for a rules change. Pruitt maintained that existing regulations are onerous for industry, that smaller, more fuel-efficient cars are less safe and that consumers want to buy SUVs and trucks despite their higher gas consumption and more polluting emissions.

When the Obama administration adopted the current national standards in 2012, its decision was accompanied by thousands of pages of supporting data, research and testing results. Magavern predicted courts will want similar documentation for any new regulations.

Although auto makers signed on to the fuel standards, with Trump’s election they saw an opening to renegotiate terms, hoping to gain some grace time for conforming to the stricter rules. Some manufacturers now say they never advocated for the wholesale rollbacks Washington is proposing.

A group of automakers sounded a conciliatory note Thursday, calling for “substantive negotiations to begin.”

“We urge California and the federal government to find a common-sense solution,” the trade groups Auto Alliance and Global Automakers said in a statement.