A California regulator is threatening to enact tough new rules in response to the Trump administration’s proposed rollbacks of vehicle emission standards.

Those rules could even include a ban on fossil fuel–based engines.

California — thanks to a waiver from the federal government — has some of the strictest emissions standards for vehicles in the county, but that status is jeopardized by proposed rules from the Trump administration that would scale back emissions standards.

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California has already sued over the proposal, and California Air Resources Board Chairwoman Mary Nichols said the board is willing to pursue other requirements to offset the increase in pollution that would arise from the emissions rollback.

“If we lose the state vehicle standards, we have to fill up the gap with other measures,” Nichols said Thursday, according to reporting from Bloomberg. “We will be faced with dramatic alternatives in terms of tighter, stricter controls on everything else, including movement of vehicles and potentially looking at things like fees and taxes and bans on certain types of vehicles and products.”

“That might mean, for example, tougher requirements for low-carbon fuels, looking at tighter health-protective regulations on California refineries, doubling down on our enforcement efforts on mobile and stationary sources — and might lead to an outright ban on internal combustion engines.”

The Trump administration's rollbacks have been opposed by automakers, who told the Environmental Protection Agency (EPA) they preferred standards that “continue improving the fuel economy of gasoline powered vehicles at historic rates and policies that support American leadership in zero emissions vehicles.”

California has long fought against the Trump administration’s proposal to weaken the national car emissions standards that were first established under the Obama administration.

California has the authority to set its own greenhouse gas rules for cars under the Clean Air Act and a waiver granted by the Obama administration. Thirteen states follow California’s standards for cars sold in their borders, representing about 40 percent of the nation's vehicle market.

California is aligned with the current federal rules, thanks to regulators’ previous negotiations. But the state filed a suit against the federal government in April pushing for more details about why the rollback was proposed.