In short, Mr. Trump’s efforts to bring work back to the United States could eliminate some jobs that are already here.

“Altering Nafta could fundamentally change the production of the economy — for the U.S., as well as for Mexico — and that will be very disruptive,” said Swati Dhingra, an economist at the London School of Economics. “Many of the policies being proposed could end up hurting the people who are being left behind.”

Big Ass Fans could wind up paying more for motors it imports from Mexico. It could lose sales to Canada and Mexico, now its two largest export destinations, and the destinations for more than a third of American exports over all.

“If we get into a trade war, that could significantly impact our U.S. production,” said Paul Lauritzen, the company’s vice president for manufacturing. “It just seems like the Trump guys are so focused on meeting campaign promises that they have failed to understand the reality of manufacturing and the global supply chain.”

In the world economy as depicted by President Trump, a product made in Mexico and sold on American shelves represents a theft. Such wares should have been forged in the United States, using American hands.

In this spirit, Mr. Trump first threatened to kill Nafta, and later agreed to the renegotiation getting underway. He has vowed to slap tariffs on a range of Chinese goods including steel. He has accused his predecessors of destroying American factory jobs by assenting to a series of abominable trade deals.