That means the process is now being run 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 staff members, people working on the plan say it is 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 the 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.
A career staff member at the E.P.A., speaking on condition of anonymity, said the numbers, the public comments and the analysis were at odds with what the White House wanted to do.
The White House official called the staff departures irrelevant and said that the rule was near completion. While acknowledging that a major change such as this takes time, the official said that people who were opposed to the rule, including some in the automotive industry, were starting to worry that the Trump plan was 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.