Automakers have long described that possibility as the worst-case outcome for them, forcing them to manufacture cars that meet two separate sets of standards. Opponents of the proposed Trump rule say they hope that possibility will persuade the administration to walk back the proposal before it is finalized.
In recent weeks, automakers and officials from California have held numerous meetings with Trump administration officials, both in Sacramento and in Washington, in an effort to broker a deal on a new proposal that would avoid the legal clash and the prospect of a split into two markets.
Automakers have also spoken publicly in support of sticking, in some form, to more stringent car standards. “We support increasing car standards through 2025, and are not asking for a rollback,” wrote Bill Ford, executive chairman at Ford Motor, in a blog post this year.
“It has been made very clear that excluding the state of California and freezing fuel economy standards for the better part of decade won’t prove fruitful,” said Senator Thomas Carper of Delaware, the ranking Democrat on the Senate Environment Committee. “President Trump should recognize the opportunity here and take yes for an answer, rather than try to push extreme and unwanted standards through.”
But people close to the Trump administration said that any future compromise was unlikely. “I can’t envision a scenario where President Trump and Jerry Brown would reach agreement over greenhouse gas emissions,” said Thomas J. Pyle, who advised Mr. Trump’s campaign and transition on energy policy, and heads the Institute for Energy Research, a group which advocates fossil fuel use.
Mr. Pyle said he was unworried that the ultimate outcome could be the imposition on American automakers of more regulations, not fewer. “If that’s the outcome, the automakers will figure out something to make it work,” he said.