The Trump administration announced Thursday that it is revoking California’s waiver to issue its own vehicle emission standards.

California’s Clean Air Act waiver was issued by the Obama administration in 2013 and allowed the state to set emission requirements for new automobiles sold in the state independent of the federal EPA’s promulgated requirements. Since automakers do not build different vehicles for different state markets, this waiver has effectively allowed California to enforce stricter emission standards on new consumer vehicles sold across the country. In addition, 14 other states and the District of Columbia have chosen to adopt California emission standards as their own state standards.

In a statement announcing the proposed rulemaking, the Environmental Protection Agency said that the new rule will “enable the federal government to provide nationwide uniform fuel economy and greenhouse gas emission standards for automobiles and light duty trucks.” Announcing the decision via Twitter, President Donald Trump said that “the Trump Administration is revoking California’s Federal Waiver on emissions in order to produce far less expensive cars for the consumer, while at the same time making the cars substantially SAFER.” He did not further elaborate on how relaxing pollution emissions standards would lead to safer vehicles.

California governor Gavin Newsom and Attorney General Xavier Becerra issued a statement denouncing the proposal, calling it a “desperate plan” and saying the rulemaking is motivated by a “political vendetta” against the state. Both have already vowed to challenge the law in court, as have environmental activist groups like the Sierra Club.