The Environmental Protection Agency declined to comment on the matter. But in a speech Tuesday to the National Automobile Dealers Association, EPA Administrator Andrew Wheeler made his intentions clear.

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“We embrace federalism and the role of the states, but federalism does not mean that one state can dictate standards for the nation,” he said.

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Already, 13 states and the District of Columbia have vowed to adopt California’s standards if they diverge from the federal government’s, as have several major automakers. California leaders on Tuesday said they will fight any challenge to their autonomy.

“While the White House has abdicated its responsibility to the rest of the world on cutting emissions and fighting global warming, California has stepped up,” Gov. Gavin Newsom (D) said. “It’s a move that could have devastating consequences for our kids’ health and the air we breathe, if California were to roll over. But we will not.”

Echoing the governor, state Attorney General Xavier Becerra, who has sued the Trump administration on a range of issues, vowed to head back to court, saying California’s clean car standards are “achievable, science-based, and a boon for hard-working American families and public health.”

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The official announcement had been scheduled for Wednesday, during President Trump’s trip to California, but after the news broke Tuesday, the administration postponed the policy rollout by at least a day.

Trump’s move is likely to be unpopular nationwide and in California, with Americans widely supportive of stricter fuel efficiency standards. A Washington Post-Kaiser Family Foundation poll released Friday found 66 percent of Americans oppose Trump’s plan to freeze fuel efficiency standards rather than enforce the Obama administration’s targets for 2025.

A nearly identical 67 percent majority says they support state governments setting stricter fuel efficiency targets than the federal government.

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Among Californians, the survey found 68 percent oppose Trump’s relaxation of mileage standards, while 61 percent support California’s stricter standards.

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Emissions from transportation, including cars and trucks, are the largest single source of greenhouse gases in the United States.

The standoff began last year, when the EPA and Transportation Department proposed taking away California’s waiver as part of a rule that would freeze mileage standards for these vehicles at roughly 37 miles per gallon from 2020 to 2026. The Obama-era standards had required these fleets to average nearly 51 mpg by model year 2025.

In July, California forged an agreement with four companies — Ford, Honda, Volkswagen and BMW of North America — under which they pledged to produce fleets averaging nearly 50 mpg by model year 2026. The Justice Department has opened an inquiry into whether the accord violated antitrust law.

One of the central arguments in the White House’s proposal is that the 1975 Energy Policy and Conservation Act gives only the federal government the right to set fuel standards, said the two senior administration officials, who spoke on the condition of anonymity because the announcement was not yet public.

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By seeking to strip California of its autonomy, Trump officials are forcing auto companies to choose whether they will side with the state or with the federal government. As part of July’s deal with the California Air Resources Board, the four carmakers agreed to support the state’s right to set its own tailpipe standards.

Environmentalists promised to join California in its legal opposition.

“There’s nothing in the Clean Air Act or EPA regulations providing for this unprecedented action,” Martha Roberts, a senior attorney at the Environmental Defense Fund, said in an interview. “The legislative history is explicit about broad authority for California. This is very well established legal authority that’s firmly anchored in the Clean Air Act.”

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Marlo Lewis, a senior fellow at the conservative Competitive Enterprise Institute, predicted in an interview that the move actually could make it easier for automakers to embrace the White House’s proposed rollback of gas mileage standards.

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“The only reason the automakers are not on board with Trump is because they’re afraid of the retaliation from California if Trump loses,” Lewis said.

It is unclear who would prevail in a legal fight over California’s waiver. The state’s air regulators have consistently argued that they are limiting carbon dioxide emissions from vehicles, rather than overtly setting mileage standards.

Margo Oge, who directed the EPA’s Office of Transportation and Air Quality from 1994 to 2012, said in an interview that California can make a strong case that it needs to curb these pollutants because climate change worsens ozone, which helps create smog.

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“California has demonstrated that by getting a greenhouse gas emissions waiver, it can also reduce ozone pollution, because the data is very strong,” she said.

But even Obama administration officials acknowledged that efforts to curb CO2 emissions from autos are inextricably linked to stricter mileage standards. The 2010 rule published by EPA and the National Highway Traffic Safety Administration noted that nearly 95 percent of emissions from cars and light trucks stem from motor fuel combustion.

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Auto industry officials said they continue to hope that federal and state officials can compromise on a single national standard, despite no evidence of a deal in sight.

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“Automakers have said many times that we support year-over-year increases in fuel economy standards that align with marketplace realities,” said Gloria Bergquist, a spokeswoman for the Alliance of Automobile Manufacturers, “and we support one national program as the best path to preserve good auto jobs and keep new vehicles affordable for more Americans.”

In an interview with The Post last week, Wheeler said the Trump administration plans to separately finalize scaled-back mileage standards for the nation’s autos by the end of the year. He said he remained optimistic that the industry would embrace the latest version of the rollback. “I’m still hopeful that people will see that the changes we made from the proposal to the final [rule], that everyone would get on board and be supportive of what we’re doing,” Wheeler said.

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California’s long-standing ability to write its own emissions standards has seldom been questioned in Washington. In part, that’s because of the history that led to the state’s unique authority.

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Smog in Los Angeles had become crippling at times throughout the 1950s and into the 1960s. As scientists focused on motor vehicle exhaust as a key culprit, state officials worked to develop the nation’s first vehicle emissions standards in 1966.

The following year, the state’s new Republican governor, Ronald Reagan, established the California Air Resources Board to undertake a statewide effort to address widespread air pollution.

As it crafted landmark clean-air legislation for the country, Congress granted California special status, saying the state could request a “waiver” to require stricter tailpipe standards if it provided a compelling reason for why they were needed. The auto industry, then as now, expressed concern over the idea of having to meet different standards in different states, but California eventually prevailed.

Congress has repeatedly reaffirmed that right. And in 1977, lawmakers said other states could legally adopt California’s stricter car emissions standards.

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Over time, emissions control strategies first adopted by California — catalytic converters, regulations on oxides of nitrogen, and “check engine” systems, to name a few — have become standard across the country.

In late 2007, the George W. Bush administration denied California a waiver on the grounds that capping carbon dioxide emissions did not address a specific air pollution problem for the state.

“The Bush administration is moving forward with a clear national solution, not a confusing patchwork of state rules, to reduce America’s climate footprint from vehicles,” said Stephen L. Johnson, the EPA’s administrator at the time.