California was the catalyst for the national drive to improve fuel economy through the privileged position federal law grants the state to drive environmental innovation. Facing the endemic air pollution challenges that wrapped Los Angeles in smog, the state began regulating auto tailpipe emissions in 1966––one year before the first major federal clean air legislation. Recognizing the state’s pioneering role, the 1970 Clean Air Act (signed by Republican President Richard Nixon, a Californian) allowed California, alone among the states, to obtain EPA waivers to set its own vehicle emission standards if they were at least as stringent as federal requirements. In 1977, Congress allowed other states to adopt California’s standards, rather than the national rules.

The EPA has granted California over 100 of these waivers, which have driven an array of pollution-reduction gains. That list, as Richard Frank, an environmental law professor at the University of California (Davis), recently recounted for a State Senate committee, includes the first limits on emissions of hydrocarbons and carbon monoxide and the development of the catalytic converter and the “check engine” light diagnostic system. California, Frank notes, has repeatedly provided “a model for other states, the nation and other countries.”

In 2002, California lit the long fuse for Obama’s fuel efficiency agreements when it passed landmark legislation requiring vehicles to limit their emissions of greenhouse gases-which effectively mandated the manufacturers to improve mileage performance. About a dozen other states adopted the standards, but the George W. Bush EPA blocked the California rules by denying them, for the first time, a waiver. That fight was in court when Obama took office, but it became moot when he granted the California waiver, and effectively incorporated its fuel efficiency standards through 2016 into his first national agreement with the auto companies. The process repeated in 2012: California approved new state standards through 2025, Obama granted them a waiver and essentially merged the state rules into new national rules.

Obama’s second agreement with the auto companies required the EPA to conduct a mid-point review of the standards to determine if it was technically feasible to meet the higher requirements set for the 2022-2025 model years. Just before he left office, his EPA determined it was and finalized their approval of the 2025 requirements. But in late February, the Alliance of Auto Manufacturers formally asked the Trump EPA to reconsider that review. Most observers see that as the first step toward the industry challenging the fuel efficiency mandate itself for the 2022-2025 years-and most expect Pruitt to support that request.

Pruitt’s problem is California’s second waiver from Obama: the California Air Resources Board says it provides the state authority through 2025 to implement its own carbon emission rules, which match the existing federal vehicle fuel economy standards.* That means if Pruitt and the auto manufacturers truly want to roll back Obama’s auto efficiency standards, they will need to also rescind California’s waiver for its own rules––especially since about a dozen states, cumulatively representing about one-third of all new car sales, have adopted the state’s regulations.