With his proposal to roll back fuel economy and emissions standards for cars and trucks Thursday, President Donald Trump drew a line in the California sand. The nation’s most populous state wasted no time crossing it.

“California will fight this stupidity in every conceivable way,” California Governor Jerry Brown said in a tweet.

The dispute — which rattled automakers, among others — centers on an element of the Trump proposal that would revoke California’s authority under the Clean Air Act to set rules more stringent than the federal ones limiting tailpipe greenhouse gas emissions as well as an electric-vehicle sales mandate.

The state has since 1970, when smog in Los Angeles became a rallying cry for the nascent environmental movement, employed tailpipe emissions rules that have been tougher than the federal government’s.

“For 48 years — since one of my heroes, then-Governor Ronald Reagan, requested it — California has had a waiver from the federal government to clean our own air,” action-movie star and former California governor Arnold Schwarzenegger tweeted. “If the president thinks he can win this fight, he’s out of his mind.”

In a desert state long at war with the whims of Mother Nature, emissions regulations are not just legal or regulatory issues.

“There’s a great irony in Trump releasing this plan just as California is burning,” said Ann Carlson, a University of California at Los Angles law professor. “We’re already seeing catastrophic forest fires as a result of climate change, and the administration’s answer is that everyone should drive a Hummer.”

The rollback is “especially hard to take,” Carlson said, because Trump officials invoked concern for human life lost in traffic accidents to justify it, while conspicuously failing to show the same concern for fire victims.

According to Trump officials, freezing the requirements will slow the pace at which new cars are getting more expensive, and so, allow people to replace older and less-safe vehicles more rapidly.

Why Trump Attacks California’s Anti-Pollution Powers: QuickTake

If he can undermine the waiver, Trump would destroy the state’s ability to require manufacturers to sell zero emission cars. ZEV’s are the state’s best available tool, Carlson said, to limit not just greenhouse gases that fuel climate change, but also pollutants like ground-level ozone which contribute to smog. Some of the nation’s worst smog, she said, continue to plague both Los Angeles and the state’s San Joaquin valley.

ZEVs need to make up as many as 40 percent of sales by 2030 if California is to meet its CO2 reduction targets, according to estimates by the state’s Air Resources Board staff.

But on a call with reporters Thursday, Bill Wehrum, the assistant administrator for the Environmental Protection Agency’s Office of Air and Radiation, said the agency only wants to limit California’s right to regulate greenhouses gases, and not pollutants like ozone.

“There’s pretty strong evidence that Congress intended the federal government to be the primary regulator” for both fuel economy and emissions, he said, adding that the Trump team hopes Washington and Sacramento can still negotiate a unified set of emissions regulations. “It makes no sense in the world to have a two-car world.”