When executives from the Big Three automakers went to the White House last spring to ask for more lenient emissions rules, their prospects looked rosy. President Trump seemed eager to roll back regulations, and Scott Pruitt — a friend of industry — was newly in control at the Environmental Protection Agency.

Now, automakers are coming to terms with a startling reality: Be careful what you ask of President Trump, because he could go further than you expect.

The E.P.A.’s plan, which is likely to be formally launched in coming days, is widely expected to loosen regulations on greenhouse gas emissions and fuel economy well beyond what the automakers themselves had sought.

In addition, according to a White House official with knowledge of the plans, the Trump administration is expected to formally urge California — which has special authority under the Clean Air Act to set its own air pollution laws — to go along with the rollback, something the state has already said it won’t do. That could spark a legal battle between the federal government and California, potentially dividing the United States into two automobile markets, each with a different rule book governing pollution and gas mileage.