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WASHINGTON — As the Trump administration prepares to drastically weaken Obama-era rules restricting vehicle pollution, nervous automakers are devising a strategy to handle their worst-case scenario: a divided American auto market, with some states following President Trump’s weakened rules while others stick with the tougher ones.

The effort is increasingly urgent because the Trump administration has now settled on the key details of its rollback plan, according to two people familiar with the matter. The new rules would all but eliminate the Obama-era restrictions, essentially freezing standards at about 37 miles per gallon, compared to 54.5 miles per gallon required by the current rules. The policy makes it a near-certainty that California and 13 other states, collectively representing roughly one-third of the United States auto market, will keep enforcing the stricter rules, splitting the national auto market in two.

Although Mr. Trump has billed his rollback as a boon to the auto industry, automakers say the split-market outcome would be a logistical and financial nightmare for them. Their current strategy centers on selling completely different kinds of cars in different parts of the country — chiefly hybrids or electric vehicles in states like California, and less efficient SUVs (which American car buyers love) in other states.

But automakers recognize it would be easy for some buyers to simply cross state lines to buy what they want.