The Trump administration on Wednesday (September 18) announced plans to revoke a California rule requiring automaker build cleaner cars.
The president tweeted he was "revoking California’s Federal Waiver on emissions in order to produce far less expensive cars for the consumer."
The move will set off a massive legal battle between the state and the federal government.
California has for decades received a federal waiver granting the state the right to set emissions standards higher than the rest of the nation.
And in doing so, the most populous state in the U.S. wields enormous power in the sorts of cars that get built for American consumers.
Other states have adopted many of California's standards.
But the Environmental Protection Agency under Donald Trump has tried to roll back environmental protections and emissions standards.
The president on Wednesday argued he is standing up for American automakers, and U.S. jobs, tweeting: "Automakers should seize this opportunity because without this alternative to California, you will be out of business."
It's unclear whether carmakers will side with the Golden State or the White House.
A statement from a group representing the industry said they were reviewing the Trump's waiver recommendation.
California has vowed to challenge the Trump administration effort, arguing that the United States has an obligation to protect the environment for future generations.