LOS ANGELES — State prosecutors from California to Massachusetts blasted the Trump administration Thursday for proposing weaker auto fuel-efficiency standards they said would imperil clean air and increase greenhouse gases.

Months after they preemptively sued to block anticipated efforts by the Environmental Protection Agency to roll back mileage regulations, Democratic attorneys general vowed to continue their fight in the courts.

“The earth is not flat, and climate change is real,” California Attorney General Xavier Becerra said, as he connected global warming to the deadly wildfires burning out of control throughout the state. “Can someone please inform the folks at the White House and our federal government of those facts?”

Becerra also promised another lawsuit if the administration makes good on what he called “arbitrary and capricious” plans to revoke a long-standing waiver allowing California and other states to set their own stricter auto emissions standards. At least 12 other states and the District of Columbia follow California’s rules.

Officials in the Trump administration said their actions would make autos more affordable and that would make roads safer because more motorists would be driving newer cars with the latest safety features.

Becerra and attorneys general from 16 other states sued in May to stop the EPA from scrapping standards that would have required vehicles by 2025 to achieve 36 miles per gallon in real-world driving, about 10 miles over the existing standards. The Trump proposal would freeze standards at 2020 levels when vehicles will be required to hit an average of 30 miles per gallon in real-world driving.

States that joined the lawsuit said the change would end up costing more money at the pump because vehicles won’t go as far on a gallon of gas, and more misery for those suffering pollution-exacerbated maladies such as asthma.

“This has to be absolutely one of the most harmful and dumbest actions that the EPA has taken,” Massachusetts Attorney General Maura Healey said. “It’s going to cost drivers here and across the country hundreds of millions of dollars more at the pump.”