A last-minute push by automakers appears unlikely to sway the Trump administration from abandoning President Barack Obama’s signature climate policy to improve mileage standards for cars and light trucks, two government officials said Friday.

The administration’s plan to freeze federal fuel-efficiency requirements for six years and end California’s authority to set its own standards has injected uncertainty into the auto market and raised the prospect of a drawn-out legal fight between federal officials and the nation’s biggest state.

The Environmental Protection Agency and the Transportation Department are poised to finalize a proposal this summer that would set federal car standards at roughly 37 miles per gallon, rather than raising them to nearly 51 miles per gallon for 2025 models. The rule would also revoke California’s existing waiver to set its own rules under the Clean Air Act, a practice the federal government has sanctioned for decades.

On Thursday, 17 U.S. and foreign firms sent a letter to both President Trump and California Gov. Gavin Newsom (D), urging them to “resurrect” talks to avoid harming the industry and American consumers. They warn that only a nationally agreed-upon set of rules would avert “an extended period of litigation and instability, which could prove as untenable as the current program.”

But White House officials rebuffed the automakers’ request Thursday, saying there was no prospect of further negotiation with the California Air Resources Board (CARB). The two government officials, who were briefed on the discussions, spoke on the condition of anonymity to describe internal deliberations.

“As we acknowledged earlier this year, CARB failed to put forward a productive alternative, and we are moving forward to finalize a rule with the goal of promoting safer, cleaner and more affordable vehicles,” said White House spokesman Judd Deere in an email.

Administration officials say that the nation needs to readjust the emissions targets because consumers prefer bigger and less fuel-efficient vehicles than regulators initially envisioned and that keeping them in place will spur Americans to drive older, less safe vehicles.

But California leaders show no signs of budging.

“Despite the White House’s rejection of the automakers’ appeal, we stand with those automakers, other states, and environmental leaders in pushing for one national standard — one that doesn’t backtrack on the progress states like California have made in protecting the climate and our kids’ health,” Newsom said in a statement Friday.

In 2009, the Obama administration reached an agreement with automakers and officials in California to establish the first-ever carbon standards for the vehicles. Limiting cars’ carbon output and improving fuel efficiency reduces the amount of carbon dioxide released into the atmosphere, helping to curb the pace of climate change.

In February, CARB officials said the administration had broken off communications before Christmas and had neither responded to the state’s proposals nor offered one of its own.

An aide to California Attorney General Xavier Becerra (D) said that the state was still committed to defending the standards that California, automakers and the federal government agreed to back in 2009. Thirteen states and the District of Columbia have signed on to adopting whatever tailpipe standards California sets.

In their letter to the California governor, the automakers wrote, “We know that reaching an agreement has been challenging, but the stakes are too high and the benefits too important to accept the status quo.”

Trump officials have often framed their deregulatory agenda as an effort to create more certainty for U.S. businesses and to hand more power back to individual states instead of the federal government.

But some critics argue that the administration’s tailpipe proposal would accomplish neither of those goals. California’s power to set its own standards dates to 1967 legislation and has been reaffirmed every time Congress amended the law.

“The Trump EPA’s profoundly cynical version of states’ rights does not include the fundamental rights of states — long guaranteed under the Clean Air Act — to protect millions of residents from harmful tailpipe pollution,” said Chester France, a consultant for the Environmental Defense Fund and a former EPA official.

Critics say the proposed freeze would benefit the oil and gas industry and cost consumers more at the pump. They also warn that a legal battle with California could result in a sweeping upheaval in the nation’s automotive market, should carmakers eventually have to meet different standards in different states.

One major carmaker, Fiat Chrysler Automobiles, did not sign the industry’s letters for another round of negotiations. In a statement, the automaker said its position had not changed since last fall, when an executive testified that the company is “in favor of ongoing fuel economy improvements in the fleet” but that policymakers needed to factor in the “realities” of how the auto market had changed over time.

White House officials had lobbied automakers in late February to back whatever rule the administration finalizes, according to multiple senior administration officials, and brought some of the firms back on an individual basis to solicit additional feedback. But as it became clear that the administration was forging ahead with its original plan, many manufacturers decided to make a more public statement.

Sen. Thomas R. Carper of Delaware, the top Democrat on the Senate Environmental and Public Works Committee, urged the industry to work directly with California given the administration’s stance.

“While it is not unhelpful that the automobile industry sent this letter, we are now in the 11th hour, and I fear it won’t be long before the rubber meets the road and the administration’s reckless rollback is finalized,” he said.

Bill Becker, president of Becker Environmental Consulting, said in an email that the “Hail Mary” pass from the industry won’t make a difference.

“The automakers’ efforts are too little and too late to stop the Trump administration from eviscerating the Obama clean car standards,” Becker said. “The Trump proposal, if adopted, will contribute to tens of thousands of premature deaths and millions of incidences of serious illnesses, undermine states’ compliance with the Clean Air Act, and impose serious impacts on businesses.”