California officials have discerned a chilling signal that the Trump administration may be willing to halt the state’s unique authority to impose its own vehicle emission rules—a move that could undercut its pioneering effort to battle climate change.

The threat arose during the confirmation hearing for Scott Pruitt, Trump’s choice to head the Environmental Protection Agency. Pruitt was asked by California Democratic Sen. Kamala Harris if he would pledge to continue the EPA’s decades-long policy of granting California waivers from the federal Clean Air Act, giving the state the right to set its own more stringent clean air standards. Pruitt—who as Oklahoma’s attorney general sued the EPA more than a dozen times—refused to commit to continuing California’s authority, instead saying he would have to study the issue.

The waivers have been the bedrock on which much of California’s climate change goals stand. The state’s emissions from passenger cars and light trucks have been reduced by more than 30 percent since 2009, when California expanded its use of the air quality waiver to regulate greenhouse gas emissions.

The ability of the state to chart its own course has brought California to a place of national and international leadership in combating climate change, often without the partnership of recalcitrant elected officials in Washington. In many cases, California’s regulations have become the de facto federal standards.

Fran Pavley, the recently retired Democratic state senator who authored most of California’s climate change legislation, said in an interview that the state would “absolutely not” have been able to achieve the current greenhouse gas reductions without waivers.

Losing that flexibility under the Trump administration would be a crushing blow to more than a decade of carefully crafted policies, which stitched together across multiple state and local agencies, aim to reduce California’s carbon footprint and dramatically reshape how the state generates and uses energy.

The clean air waivers also play a critical role in improving public health across California—and although they’ve helped clean pollutants from the air, large parts of the state still have air quality that violates federal ozone thresholds.

“There is no way that the 10 million Californians living in areas with smoggy air will be able to breathe healthy air without the state keeping (waiver) authority,” said Bill Magavern, policy director for the Coalition for Clean air.

Since 1967, California has requested more than 100 waivers on an array of air quality issues, ranging from requiring more advanced engine diagnostic information on heavy-duty trucks to emissions standards for forklifts. All but one was approved. That pushback came as George W. Bush was leaving office, and the state’s application was granted in Barack Obama’s first term.

Dave Clegern, a spokesman for the California Air Resources Board, said the state does not have any waiver requests pending—and that emission standards for passenger cars and light trucks are in place until 2025.

The Trump administration’s intentions are not yet clear—President Trump has described himself as an environmentalist, but also once tweeted that global warming was a Chinese hoax to stymie competition from U.S. manufacturers. Should a Pruitt-run EPA elect not to grant waivers to California, the state would likely mount a legal challenge bolstered by 50 years of precedent.

“I think it would be a heavy lift to say no now,” said David Pettit, a senior attorney in the Natural Resources Defense Council’s Southern California Air Program. “The EPA can’t do something that is arbitrary or capricious. If you show a judge a 1,000-page administrative record, that judge is going to ask EPA what has changed.”

He said that because California’s right to seek an EPA waiver is written into statute, the “nightmare scenario” would come into play should Congress undertake to rewrite the Clean Air Act.

“There have been rumblings,” Pettit said.

But some conservatives, who contend that regulations often saddle businesses with unfair burdens, welcome the notion of eliminating the right of states to make their own rules. Former state Assemblyman Tim Donnelly recently wrote a column for the alt-right Breitbart website supporting dismantling both the state air board (CARB) and the EPA.

“But now, CARB could be on the ropes for a change,”  Donnelly wrote. “If President Trump’s pick for EPA picks a serious fight with CARB, this could be the first ray of hope for the long-suffering Americans living behind enemy lines in the Socialist Republic of California.”

Expect California’s leaders to strike back at any such move. Since the November election, the state has girded for a fight, bringing aboard a legal hired gun, former U.S. Attorney General Eric Holder, to look out for the state’s interests. California’s new attorney general, Xavier Becerra, also intends to defend state policies from erosion by federal decree.

California’s right to set its own clean air standards dates back to the mid 1960s, when the federal Clean Air Act was written. In acknowledgement of the state’s smog crisis—when the mountain backdrop of Los Angeles resembled a brown smudge— California was the only state granted the ability to seek a waiver from federal regulations so that it could set stricter emission standards.

“We needed exceptional regulations in order to meet federal standards,” Clegern said. “We have unique geography, a large population and millions of vehicles on road of all ages and conditions. The one-size-fits-all national rule did not get us anywhere.”

In many instances, policies California put in place through waivers were adopted as national and even international standards. For example, the state used waivers to mandate the adoption of unleaded gasoline and to require that vehicles be fitted with catalytic converters.

“An engineer from China told me several years ago, ‘If it weren’t for California, we wouldn’t have catalytic converters in China,’” Pavley said.

Pavley was in the delegation that went to Washington in 2008 to negotiate when the EPA issued its only waiver denial. She said that today California has a target on its back with the Trump administration, but the battle isn’t just about one state.

“I can’t believe that Americans and other states don’t care about clean air,” she said.