The Trump administration is planning a proposal to block California regulators from enforcing their own emissions standards for vehicles sold in the state.

Bloomberg News reported Monday that the proposal will be part of a regulation the Environmental Protection Agency (EPA) and the National Highway Traffic Safety Administration (NHTSA) will jointly propose in the coming days to freeze or reduce federal greenhouse gas emissions and fuel efficiency rules for cars.

The proposal would set up a battle with California over whether the Clean Air Act allows its decades-long aggressive crackdown on car emissions. The fight is almost certain to go to court if the administration pursues it.

The rules have become part of California’s environmental identity, as well as part of its efforts to clean the air in Los Angeles and other heavily polluted areas.

The Obama administration permitted California to set its own greenhouse gas emissions for cars in 2009, giving it a waiver under the Clean Air Act. Numerous other states now follow California’s standards.

Under Obama, the EPA and NHTSA decided to negotiate with California so that the country retains one nationwide standard, even though California had the authority to institute its own.

Separately, the Golden State uses its waiver to require car companies to sell a certain amount of electric vehicles in the state. That authority would also be revoked with the Trump administration’s action.

The EPA and NHTSA revealed in a regulatory notice Friday that its upcoming proposal to reduce vehicle efficiency and emissions standards will be dubbed the “Safer and Affordable Fuel Efficient Vehicles Rule,” indicating that administration officials will likely argue that stricter standards would compromise safety.

Then-EPA head Scott Pruitt formally declared in April that the Obama plan to make emissions and efficiency standards stricter through 2026 is not appropriate. It was the first step toward potentially rolling the standards back.

The agencies are expected in the coming days to float a proposal with a handful of ideas, including various levels of looser rules through 2026 and freezing the standards in 2020 with no additional ramping up.