Four major carmakers have struck a deal with California on fuel efficiency rules, despite an attempt by the Trump administration to strip the state of the right to set its own standards to fight climate change.
California negotiated with Ford, Honda, Volkswagen and BMW in secret.
Along with a dozen other US states, it has vowed to enforce stricter Obama-era emissions standards.
President Trump wants to roll back federal rules on car emissions.
Last year, his administration proposed a rule to axe tougher mileage and greenhouse gas emissions requirements enacted by his predecessor.
It also proposed revoking California's right to impose state emissions standards or require more electric vehicles.
The White House has said "the federal government, not a single state, should set this standard. We are moving forward to finalise a rule for the benefit of all Americans".
California Governor Gavin Newsom announced the agreement on Thursday. "California, a coalition of states, and these automakers are leading the way on smart policies that make the air cleaner and safer for us all," he said.
"I now call on the rest of the auto industry to join us, and for the Trump administration to adopt this pragmatic compromise instead of pursuing its regressive rule change. It's the right thing for our economy, our people and our planet."
California accounts for about 12% of US vehicle sales, and if the federal administration recognises the deal, it would allow carmakers to operate under one set of rules across the country.
"These terms will provide regulatory stability, preserve vehicle affordability for customers, reduce compliance costs and result in increased environmental benefits," the manufacturers said in a statement.
The California agreement, which is voluntary, is slightly less restrictive than the Obama standards and can apply to vehicles sold nationwide.
Under the framework, by 2026, new models would meet a standard of 50 miles per US gallon (4.7 litres per 100km), against the current 37 mpg level.
Increased fuel efficiency means vehicles burn less petrol and emit fewer polluting greenhouse gases into the atmosphere.
Last year, the Trump administration proposed revoking California's right to impose state emissions standards or require more electric vehicles.