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WASHINGTON — The White House, blindsided by a pact between California and four automakers to oppose President Trump’s auto emissions rollbacks, has mounted an effort to prevent any more companies from joining California.

Toyota, Fiat Chrysler and General Motors were all summoned by a senior Trump adviser to a White House meeting last month where he pressed them to stand by the president’s own initiative, according to four people familiar with the talks.

But even as the White House was meeting with automakers, it was losing ground. Yet another company, Mercedes-Benz, is preparing to join the four automakers already in the California agreement — Honda, Ford, Volkswagen and BMW — according to two people familiar with the German company’s plans.

Mr. Trump, described by three people as enraged by California’s deal, has demanded that his staff members step up the pace to complete his plan. His proposal, however, is directly at odds with the wishes of many automakers. They fear that the aggressive rollbacks will spark a legal battle between California and the federal government that could upend their business by splitting the United States into two car markets, one with stricter emissions standards than the other.