On Wednesday, a coalition of free-market groups including the Competitive Enterprise Institute urged Mr. Pruitt to take California on. “It is time for the E.P.A. to act,” the groups said. If the agency did not act quickly, the groups said, “people across the state of California will be facing unrealistic and costly mandates which threaten their basic right to choose.”

President Trump has also spoken about rolling back the efficiency rules, known as Corporate Average Fuel Economy, or Cafe. “I’m sure you've all heard the big news that we’re going to work on the Cafe standards so you can make cars in America again,” Mr. Trump said at a Detroit auto research facility in March last year. “We want to be the car capital of the world again. We will be, and it won’t be long.”

The rules, aimed at cutting tailpipe emissions of carbon dioxide, a major contributor to global warming, were one of the two pillars of Mr. Obama’s climate change legacy. Put forth in 2012, they would have required automakers to nearly double the average fuel economy of new cars and trucks to 54.5 miles per gallon by 2025.

If fully implemented, the rules would have cut oil consumption by about 12 billion barrels and reduced carbon dioxide pollution by about six billion tons over the lifetime of all the cars affected by the regulations, according to E.P.A. projections.

The rules also would have put the United States, historically a laggard in fuel economy regulations, at the forefront worldwide in the manufacture of electric and highly fuel efficient vehicles. The United States and Canada are the only major nations that have adopted mandatory emissions standards through 2025. The European Union has only recently proposed standards for 2025 and 2030, while China has only started to work on standards for those years.

Less restrictive regulations in the United States could provide an opening for automakers to push for more lenient standards elsewhere as well, leading to the emission of more pollution by cars around the world. While sales of electric vehicles are starting to take off, they still represent barely 1 percent of global car sales. A shift among car buyers toward larger cars and trucks is already impeding progress in fuel economy.

“The concern is that automakers will go around the world basically trying to lobby regulators, saying, look, because the United States has reduced the pace, everywhere else should too,” said Anup Bandivadekar, a researcher at the International Council on Clean Transportation, a think tank that focuses on clean car technology and policy. Global carmakers “apply developments in one region to lobby for changes in other regions.”