BRUSSELS — President Trump has complained about seeing too many German cars on Fifth Avenue, and threatened heavy tariffs on the companies that produce them. There is a good chance, though, that those Mercedes-Benzes and BMWs were not only made in the United States, but made by workers who voted for Mr. Trump.

European companies have turned Alabama, South Carolina and Tennessee into auto manufacturing powerhouses in recent years, churning out cars not just for American buyers but also for export to China and Europe. Germany’s three biggest carmakers all have facilities there, and Volvo Cars, which is owned by a Chinese company but based in Sweden, began producing at a new plant in South Carolina just last month.

Yet being major employers in regions that voted heavily for Mr. Trump has not protected them.

With barely a peep of resistance from his own party, the president has threatened tariffs — expected to be 20 percent — on imported cars and car parts. In a prelude to such a move, he has ordered an investigation into whether the imports pose a threat to national security. Trade restrictions could be put in place within months. And if he follows through, the European Union has pledged to retaliate.

The damage would be far-reaching, draining an estimated $14 billion from the United States economy. If other countries retaliated, the cost would skyrocket to nearly $300 billion, the European Union’s Washington delegation said last week.