Seventeen states, led by California, announced a lawsuit against the Environmental Protection Agency today over its recent rollback of Obama-era vehicle emissions and fuel economy standards. The EPA decided at the beginning of April that the standards, which were set by the previous administration, were “not appropriate and should be revised.”

The lawsuit was filed in the US Court of Appeals for the District of Columbia Circuit. With it, the states — along with the California governor’s office, California attorney general, and the California Air Resources Board — argue that the EPA “acted arbitrarily and capriciously” in overturning the previous administration’s decision. Reached for comment, a spokesperson for the EPA said the agency “does not comment on pending litigation.”

The California Governor’s office argues the EPA “acted arbitrarily and capriciously”

The standards in question were drawn up in 2009 and adopted in 2012. They laid out a path for automakers to reduce overall greenhouse gas emissions by reaching an average fleet fuel economy of 54.5 miles per gallon by 2024. Since the program was charting a course that stretched out more than a decade into the future, it was written into the rules that the EPA would have to perform a “mid-term evaluation” before April 1st, 2018. This review would serve two purposes: assess whether automakers were on track, and then use that information to determine if the last section of the standards (which apply to model year 2022–2025 cars) were still feasible.

The EPA, under Barack Obama, kicked off this review process ahead of schedule in the summer of 2016 when it published an extensive 1,200-page technical assessment that analyzed whether the standards were working. In January 2017, the outgoing EPA wrapped this evaluation and determined that the bar was not set too high. In fact, it argued, automakers were overwhelmingly compliant.

“Last year, the U.S. EPA affirmed these standards were appropriate based on an extensive record of data,” the governor’s office writes in today’s statement. “On April 13, 2018, however, the U.S. EPA, without evidence to support the decision, arbitrarily reversed course and claimed that the clean car standards for model years 2022–2025 should be scrapped.” The governor’s office also argues that the federal agency “offered no evidence” in support of its decision, something some industry analysts agree with.

The Trump EPA’s decision in April did not set new standards — it simply argued that there were problems with the existing standards. In the meantime, the agency and the Department of Transportation are currently working together to craft and officially propose new standards. But the previous standards that the EPA said were inappropriate will technically remain in place until that happens.

Still, many environmental groups, and even some automakers, viewed the original standards as a key tool in fighting climate change — especially because transportation is now the largest source of CO2 emissions in the US. The EPA is reportedly looking to lower the standards, but whatever it decides, moving to revisit the standards in the first place has already turned up tension in the industry.

The lawsuit is also just the latest blow in a back-and-forth between Scott Pruitt and the state of California. When the EPA announced its decision in April, it also said it was “re-examining” a waiver that the state was granted under the Clean Air Act to set its own air quality standards. During testimony in front of Congress last week, EPA administrator Scott Pruitt said multiple times that the agency is working closely with California and CARB on the next steps in the process of dealing with the standards and the waiver, but CARB immediately disagreed with those claims.

Today, Mary Nichols, the chair of CARB, took this fight a step further. “But now Administrator Pruitt, based on no new information or facts, wants to roll back all that progress in the name of deregulation,” she says in the statement. The EPA’s recent decision, she argues, “is just the first step but it is intended to provide the legal basis for a decision that has already been made: to halt the progress that regulators and industry have made toward a new generation of vehicles. It does not withstand scrutiny and it will not stand.”

While numerous states already follow the high emissions regulations bar that California’s set for itself, even more joined it in the lawsuit announced today. “The states joining today’s lawsuit represent 140 million people who simply want cleaner and more efficient cars,” California Gov. Jerry Brown said in the announcement. “This phalanx of states will defend the nation’s clean car standards to boost gas mileage and curb toxic air pollution.”

“The evidence is irrefutable: today’s clean car standards are achievable, science-based and a boon for hardworking American families. But the EPA and Administrator Scott Pruitt refuse to do their job and enforce these standards,” California Attorney General Xavier Becerra said in the statement. “Enough is enough. We’re not looking to pick a fight with the Trump Administration, but when the stakes are this high for our families’ health and our economic prosperity, we have a responsibility to do what is necessary to defend them.”

Correction: There are 17 states involved in the lawsuit, not 18 as previously stated.