Mr. Trump directed the E.P.A. under Mr. Pruitt to craft a new, less strict set of standards. The announcement expected on Tuesday would represent the first legal step in the process.
While Mr. Pruitt’s proposal to open up the Obama rules to review isn’t expected to include specific targets, “The proposed rollback is going to be quite a significant number,” said Myron Ebell, who led Mr. Trump’s E.P.A. transition team and directs the energy and environment policy at the Competitive Enterprise Institute, a Washington research organization that questions the established science of human-caused climate change. “It will be more than a couple m.p.g.,” he said.
If the legal fight between California and the Trump administration escalates, one possibility is that the federal government might try to revoke the waiver allowing California to set its own rules. Some presidents, including George W. Bush, have considered revoking the waiver, but none have tried.
The announcement by Mr. Pruitt was not expected to include a decision on challenging the waiver.
Mr. Ebell suggested that one possible legal tactic for the Trump administration could be to announce that it will refuse to renew the current waiver on tailpipe emissions, which expires in 2025, rather than to revoke it outright. That would likely delay a court fight until California moves to set standards that go beyond 2025.
But such a move would also likely formalize, at least for the time being, two different sets of rules in the United States — the federal emissions rules, and California’s stricter ones — a logistical headache for the industry.
While California and its ally states have long followed separate smog standards, those have been easier for automakers to meet because a car can be brought into compliance by adding a catalytic converter, for example. Designing for separate mileage standards is more difficult, because fuel economy is dependent on a car’s weight and design.
A divided market could require substantially different car designs, experts say, putting the American auto industry into uncharted territory. It remains unclear how the issue might be resolved. One possibility is that two very different auto markets emerge, one with cleaner cars generally along the coasts, and another with more polluting cars concentrated in Middle America. On the other hand, automakers might also opt to generally adhere to the stricter California standards nationwide, blunting the impact of any Trump administration rollback of federal rules.