Assuming the Trump administration’s plan is finalized and survives legal challenges, America’s cars and trucks would emit as much as an extra 321 million to 931 million metric tons of carbon dioxide into the atmosphere between now and 2035 as a result of the weaker rules, according to an analysis by the research firm Rhodium Group .

In 2018, Mr. Ebell’s group weighed in at a critical juncture in the policymaking process — one day before auto executives were scheduled to visit the White House to urge Mr. Trump to temper his rollback. “We understand that some in the automaker community have expressed concerns about your current plans to reform the program,” Mr. Ebell wrote in a May 10, 2018, letter to Mr. Trump, signed by him along with two other people who assisted with the transition.

“Their main concern is the expected legal battle with California,” they wrote. “In fact, what many liberal California politicians really want to do is ban gasoline-powered cars altogether.” Their letter went on to say: “You should dismiss this concern. We agree that in an ideal world, California would negotiate with you in good faith, but we all know that is not a reality in this current political climate.”

In an interview, Mr. Ebell said his group had succeeded in influencing the rule-making, in particular convincing the Trump administration that forcing automakers to build cleaner cars will lead to more highway accidents and deaths. Auto experts have largely debunked that assertion.

“It’s taken a long time but finally the Trump administration has come around. Lighter, smaller cars are less safe than bigger, heavier cars,” he said. “The automakers are in a hopeless position,” he added. “California is not going to negotiate. A deal that everyone can live with is not going to happen.”

The Auto Alliance, which represents some of the country’s largest automakers, declined to comment.

The newly released government correspondence also hints at the influence of fossil fuel money in the policymaking process. The New York Times reported in December that Marathon Petroleum, the country’s largest refiner, worked with a powerful fuel-industry group, American Fuel and Petrochemical Manufacturers, to run a stealth campaign to roll back car emissions standards.