Last week, the Trump administration unveiled one of its most consequential environmental rollbacks yet, a plan to let cars pollute more while stripping California of its right to set its own air-quality rules.

On Tuesday, California opened its counteroffensive, laying out in a report a scathing rebuttal that the state’s clean-air regulator, Mary Nichols, said would inform its legal and regulatory battle with Washington in the coming months and years.

The administration’s proposal “is contrary to the facts and the law,” the document says, before rebutting point by point the administration’s arguments for weakening the nation’s long-term goals for making vehicles more fuel efficient and less polluting.

The clash between California and Washington threatens to throw the U.S. auto market into disarray. Because California has the authority under the Clean Air Act to set its own air pollution rules and because a dozen other states follow its lead, the dispute could effectively split the nation’s market into two, one side adhering to stringent emissions rules set in Sacramento and the other to weaker federal standards.

As a first step, California plans to change its rules to declare that the federal government’s current, stricter auto-emission targets are the only ones that comply with state law — and not any future targets that are less strict.

That move collides with the administration’s plan to freeze Obama-era rules, which would have required automakers by 2025 to roughly double the fuel economy of their new cars, pickup trucks and SUVs to about 36 mpg. Those rules represented one of President Barack Obama’s most significant efforts to fight climate change.

The Trump proposal would freeze the standards after 2021, when new cars must average around 30 mpg. Among the government’s justifications for making the change is the argument that forcing automakers to build cleaner cars would lead to more highway accidents and deaths. Experts have questioned that claim; California disputed it as well.

Weakening the national standard “could substantially slow progress toward the emission reductions needed to address the serious threat climate change poses to California, the country, and the world, waste billions of gallons of gasoline, and cost consumer money on fuel,” the state’s report said.

California’s move would also unequivocally establish a separate emissions regime for the state and the 12 others that follow its standards, a coalition that covers more than 130 million residents and more than a third of the domestic auto market. Heavily Democratic California has emerged as a bulwark against the Trump administration’s policies.

Carmakers have balked at the prospect, calling for the Trump administration and California to come to terms to avoid a messy legal battle.

That looks unlikely.

“We’re going to do everything we must do to continue to move forward rather than backslide,” said Xavier Becerra, California’s attorney general. “We see the havoc that climate change brings to the environment and the last thing we need to do is to close our eyes to it.”

At stake is what could be President Donald Trump’s biggest climate-policy rollback, a move that could, by 2035, increase U.S. greenhouse gas emissions by an amount that exceeds what some countries, such as Austria or Bangladesh, release in a year.

The extent of the rollback, and the prospect of an all-out legal battle between the Trump administration and California, has made even some automakers uneasy. On Monday, the Alliance of Automobile Manufacturers, the industry’s main lobbying group, posted to its Twitter account the headline and link to an opinion piece titled “The Trump administration’s fuel-efficiency proposal is unnecessary and harmful.”

By Tuesday, the post had been erased. Gloria Bergquist, a spokeswoman for the Alliance, said the tweet had been a “mistake,” though there were parts of the opinion piece, published by the Brookings Institution, that the group agreed with.

California also said that it “has been, and remains, willing to consider well-founded and necessary changes to the program,” but that the headline fuel-economy targets, and their associated reductions in greenhouse gas emissions, are not negotiable.