What are the details?
California was granted a waiver to the Clean Air Act from the moment the act was first drafted in 1970. The state argued that since it was already enacting strict environmental regulations, it should be allowed to continue to do so on its own terms.
Expanding on this, the Obama administration allowed the state to regulate greenhouse gas emissions for cars on its own terms. According to the waiver, it can make these regulations as strict as it wants, as long as they are at least as stringent as federal regulations.
This change might be targeting California's waiver, but it would also affect more than a dozen other states. Although other states cannot ask for waivers on their own, if California imposes stricter standards, other states can opt to also implement those same standards above and beyond what federal law requires. Fifteen states use this rule to implement California's standards within their own borders.
Since it was passed, every president has granted California additional waivers to pass its own regulations with one exception: in 2004, former President George W. Bush turned down a request from California to impose restrictions on carbon dioxide emissions from automobiles. Former President Barack Obama would grant this waiver five years later.
This announcement wouldn't come without warning. In March 2017, former Environmental Protection Agency administrator Scott Pruitt was reportedly looking at ways to remove California's waiver, according to a report from The New York Times.
This move would be part of a larger policy change that would seek to limit regulations on greenhouse gas emissions and fuel efficiency standards, according to Bloomberg. Bloomberg cited “three people familiar with the plan” as sources.