In a potential blow to California’s fight against climate change, the Trump administration has taken aim at a deal the state made with automakers to cut greenhouse gases in car exhaust.

The U.S. Environmental Protection Agency and the Department of Transportation warned California Air Resources Board chief Mary Nichols today that the deal “appears to be inconsistent with Federal law.” The move came at the same time as a Wall Street Journal report that the Department of Justice is investigating whether four car companies broke competition law by forging their own agreement with California.

The pact was a California defense against the Trump administration’s plans to roll back Obama-era car emissions and gas-mileage rules. Trump has also threatened to stop allowing California to set its own tailpipe standards.

That revocation of California’s regulatory authority may be imminent — and would be a major setback for the state’s long-term ability to combat greenhouse gases in car exhaust.

California has vowed to fight back — mostly likely with a lengthy lawsuit. “If the administration opts to create chaos, they’ll be in court for years with us,” Craig Segall, assistant chief counsel for the California Air Resources Board, anticipating the further federal action.

In preparation for the rollbacks and the regulatory uncertainties raised by a lengthy court battle, California and the four major automakers — Honda, Ford, BMW, and Volkswagen — cut their own deal in July. California agreed to give the companies more time and flexibility to meet existing greenhouse-gas targets, and the companies agreed to voluntarily follow California’s rules.

The deal is still an unsigned, unfinalized framework. But it apparently incensed the president, provoking tweets about “the weakness of current car company executives.” News reports said the president pressured big automakers to reject California’s deal.

Alexei Woltornist, a spokesperson for the Department of Justice, declined to comment on any investigation into the four car companies and ignored an emailed question about whether the White House was involved in opening an investigation.

Pietro Zollino, a spokesperson for Volkswagen, said the company is in touch with U.S. authorities on “a number of matters” but doesn’t comment on private communications “we may or may not receive.” Ford, BMW and Honda all confirmed that they received a letter from the Department of Justice. Phil Dilanni, a spokesperson for BMW of North America, said the letter requested “information concerning a planned CARB framework agreement” with BMW and other auto manufacturers.

Mary Nichols chairs the California Air Resources Board. Photo by Carl Costas for CalMatters

Air board chief Nichols said in an emailed statement that an investigation into the auto companies is “an attempt to frighten them out of voluntarily making cleaner, more efficient cars and trucks than EPA wants.”

California Gov. Gavin Newsom said California won’t back down from the Trump administration’s efforts. “California stands up to bullies and will keep fighting for stronger clean-car protections that protect the health and safety of our children and families.” On Twitter, he addressed Trump directly, saying “Stop threatening carmakers and trying to roll back our rules.”

For a full explainer on California’s fight with Washington over these issues, please click here.

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