I am not sure how they can do this, this waiver is written into the Clean Air Act, but when has the law ever stopped Trump and Evil Minions^™:

The New York Times reports that the Trump administration will use a meeting at the Environmental Protection Agency on Wednesday to announce the revocation of California’s ability to set its own air pollution standards. The state’s authority was granted by a waiver that allows it to set pollution limits that are stricter than the federal government’s, which is now threatening the administration’s ability to roll back Obama-era standards for automobile fuel economy. This move has been rumored to be under consideration for months and sets up a legal showdown that will pit the federal government against California and the 13 states that plan to follow its lead.

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During the initial implementation of the Clean Air Act, the Golden State was suffering from extensive smog problems and was granted a waiver that allowed it to set stricter pollution standards than those under the Clean Air Act. The waiver has since given the state significant leverage in negotiations regarding national automotive pollution controls, a position enhanced by the decision of a number of states to adopt whatever standards California sets. Due to the vast size of these states’ collective economies, car companies are compelled to meet its pollution standards or generate two different products: one for California and one for the rest of the country. Most have found it easier to simply involve California in negotiations from the start.

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All of which would explain why the Trump administration would be interested in revoking the state’s waiver and why it’s already laid out arguments to justify doing so. The Times reports that this isn’t an indication that the EPA has decided what the new standards should be yet, simply that the agency is clearing the way to impose the standards when they’re ready.

But the Clean Air Act waiver mechanics are set up so that the EPA administrator must grant a waiver to any state wanting stricter standards unless the state is acting in an “arbitrary and capricious” manner or its standards don’t address “compelling and extraordinary conditions.” California would certainly have compelling arguments that climate change represents a compelling and extraordinary condition. And it’s near certain that the state would be willing to test those arguments in court.

Unfortunately, it will be a very close thing in the Supreme Court, because there are now a majority of right wing hypocrite hacks on the bench there.