On May 1 a coalition of 18 states sued President Donald Trump's administration over plans to roll back vehicle emissions standards.
The suit, which was led by California, seeks to protect standards passed under Trump's predecessor Barack Obama that would raise fuel efficiency to about 50 miles per gallon by 2025.
"The states joining today's lawsuit represent 140 million people who simply want cleaner and more efficient cars," California Governor Jerry Brown said.
"This phalanx of states will defend the nation's clean car standards to boost gas mileage and curb toxic air pollution."
California Air Resources Board Chair Mary Nichols added that the standards the states were seeking to protect were adopted in 2012 but were meant to take effect in 2022.
"They were based on the best judgment of engineers about what technology could achieve. And in fact, they are being achieved today, years ahead of the deadlines, because of the good work of the auto industry," she said.
Connecticut, Delaware, the District of Columbia, Washington, Illinois, Iowa, Maine, Maryland, Massachusetts, Minnesota, New Jersey, New York, Oregon, Pennsylvania, Rhode Island, Vermont, Virginia and Washington state have all joined the suit. Filed in the US Court of Appeals in Washington, it targets the EPA.