“This is a politically motivated effort to weaken clean vehicle standards with no documentation, evidence or law to back up that decision,” Mary Nichols, head of the California Air Resources Board, said in a statement. She argued that the move would “demolish” the nation’s shift toward cleaner cars and that “EPA’s action, if implemented, will worsen people’s health with degraded air quality and undermine regulatory certainty for automakers.”

Nichols also hinted at a potential legal fight to come.

“This decision takes the U.S. auto industry backward, and we will vigorously defend the existing clean vehicle standards and fight to preserve one national clean vehicle program,” she said. The EPA’s decision “changes nothing in California and the 12 other states with clean-car rules that reduce emissions and improve gas mileage — those rules remain in place.”

The efficiency gains that the U.S. auto fleet has made in recent decades have slowed since 2013, as gas prices dipped and the sale of pickup trucks and SUVs accelerated. In the document Pruitt signed Monday, he said EPA had been “optimistic in its assumptions and projections” about the availability of technology to meet the standards and the agency recently had received substantial input from automakers that they needed to be scaled back.

He suggested that if cleaner vehicles are too expensive, consumers will hold onto older cars, thereby lowering the overall efficiency of cars on the road.

Peter Welch, president and chief executive of the National Automobile Dealers Association, said in a statement Monday that while the group supports “continuous improvements” in reducing vehicle emissions, “Standards alone — whatever they are — won’t do the trick.”

The Alliance of Automobile Manufacturers, whose members produce 70 percent of the cars and light trucks sold in the United States, endorsed the shift. The group estimates that it would be more realistic to require the fleet to reach a miles-per-gallon target in the high 40s by 2025.

The U.S. fleet averaged 31.8 mpg for model year 2017, according to federal figures.

Alliance spokeswoman Gloria Bergquist said in an email that her members “support the administration for pursuing a data-driven effort and a single national program as it works to finalize future standards. We appreciate that the administration is working to find a way to both increase fuel economy standards and keep new vehicles affordable to more Americans.”

But two of those members, Ford and Honda, recently urged the government to maintain the current requirements but give manufacturers additional flexibility.

At the Safe Climate Campaign, Director Dan Becker projected that retaining the Obama rule would cut carbon dioxide emissions by 6 billion tons and save 12 billion barrels of oil over the lifetime of vehicles complying with these standards. “Even though automakers are pushing gas-guzzling pickups and SUVs rather than more efficient cars, it’s still the biggest step any nation has ever taken to cut global warming pollution and save oil,” he said.

Two of Pruitt’s predecessors were harshly critical.

“All they care about is undoing everything the prior administration did, and they’ll use any excuse for doing that. They don’t even have the industry itself asking for this,” said Gina McCarthy, EPA administrator under President Barack Obama and now director of Harvard’s Center for Health and the Global Environment.

McCarthy said that the standards set during the Obama era were based on extensive negotiations with states and the federal government, as well as the auto industry. “The decision I made was based on real information,” while Pruitt’s decision seemed to have no factual basis, she said.

And former EPA administrator Carol M. Browner, who helped forge the initial carbon thresholds for cars and light trucks in 2009 while serving in the Obama White House, took issue with Pruitt’s allegation that officials in California are somehow at fault, saying “this idea that California is dictating or arbitrating for the rest of the country is not accurate.”

Rather, Browner said, federal and state officials in past administrations worked to reach a compromise that gave certainty to automakers while moving the nation to embrace more fuel-efficient vehicles.

“There’s an opportunity for us to lead the global market in cleaner, more efficient cars,” she said. “But [Trump officials] are simply going to walk away from that opportunity.”

juliet.eilperin@washpost.com

Read more:

Some automakers have second thoughts on rolling back fuel efficiency rules

Pruitt says California is ‘not the arbiter” of national tailpipe standards

Even in Trump’s America. California could determine how clean your car runs