The Supreme Court won't hear a challenge to California's first-in-the-nation mandate requiring fuel producers to reduce greenhouse gas emissions.
The justices on Monday turned away appeals from fuel makers that say the law discriminates against out-of-state producers.
The mandate requires petroleum refiners and fuel distributors to make cleaner-burning fuels for the California market.
Out-of-state oil refiners and ethanol companies say the law provides an unfair advantage to in-state producers by giving a higher "carbon-intensity score" to fuels transported from other states.
The state says the law simply offers incentives for companies that make cleaner-burning fuels.
A U.S. district judge agreed with the challengers. The 9th U.S. Circuit Court of Appeals reversed, saying the law gives all fuel makers options to get their product to market.