Until a few years ago there was not much in this patch of desert 250 miles north of Mexico City but rattlesnakes, coyotes and cactus. Today, it is gleaming evidence of the country’s growing importance as an auto producer.

In 2019, BMW completed a vast factory complex here, near the city of San Luis Potosí. As spotless and modern as any in Bavaria, the plant builds luxury sedans for the United States, Europe, China and dozens of other markets.

San Luis Potosí is one of several Mexican cities that have become little Detroits, producing Volkswagens, Audis, Mercedes-Benzes, Fords, Nissans and Chevrolets. In the first nine months of this year, Mexican factories produced more than three million vehicles, of which two million were exported to the United States, according to the Mexican Automobile Industry Association.

But Mexico’s pivotal role in the global auto industry is now at risk. President-elect Donald J. Trump has threatened to impose punitive tariffs of 100 percent or higher on cars from Mexico, which would violate a trade agreement his first administration negotiated with Canada and Mexico.

VideoThe BMW factory in San Luis Potosí has 3,700 employees.CreditCredit...Bénédicte Desrus for The New York Times

The consequences for the auto industry would be profound, affecting the price in the United States of popular models like Ford Maverick pickups, Chevrolet Equinox sport utility vehicles and several variations of Ram trucks.

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