That means the process is now being helmed by Francis Brooke, a 29-year-old White House aide with limited experience in climate change policy before moving over from Vice President Mike Pence’s office last year. Given the lack of experienced senior staffers, people working on the plan say it is now unlikely to be completed before October.
At the same time, staff members at the Environmental Protection Agency and Transportation Department, which are writing the rule, say they are struggling to assemble a coherent technical and scientific analysis required by law to implement a rule change of this scope.
Several analyses by academics and consumer advocates have questioned administration’s claim of benefits to the public. An Aug. 7 report by Consumer Reports concluded that Mr. Trump’s proposed rollback would cost consumers $460 billion between vehicle model years 2021 and 2035, an average of $3,300 more per vehicle, in car prices and gasoline purchases. It also found the rollback would increase the nation’s oil consumption by 320 billion gallons.
“The numbers, public comments and real analysis are at odds with what the White House wants to do,” said one career staff member at the E.P.A., speaking on condition of anonymity.
The White House official called the staff departures “irrelevant” and said that the rule was near completion. “It is a major change, so it does take time. What we are seeing now is that people who were opposed to the rule from the beginning, including some in the automotive industry, are starting to get nervous that our plan to make cars safer and more affordable is going to succeed.”
Policy experts point out that Mr. Trump’s quest to undo his predecessor’s signature climate-change regulation despite opposition from the very industry being regulated is extraordinarily unusual. For automakers, they say, it makes more sense to try to remain globally competitive by building more sophisticated vehicles as the world market moves toward more efficient cars.
“I don’t think there is any precedent for a major industry to say, ‘We are prepared to have a stronger regulation,’ and to have the White House say, ‘No, we know better,’” said William K. Reilly, who headed the E.P.A. in the first George Bush administration.