The mills and smelters that supply the raw material, and that would directly benefit from the tariffs, have been shrinking for years. Today, those industries employ fewer than 200,000 people. The companies that buy steel and aluminum, to make everything from trucks to chicken coops, employ more than 6.5 million workers, according to a Heritage Foundation analysis of Commerce Department data.

Mr. Woltz, who is based in North Carolina, counts himself among hundreds of specialized businesses that will bear the brunt of the tariffs.

He pays around $20 an hour on average, and he has been able to increase his payroll despite stiff competition from abroad. If you have seen a bridge being hoisted over a highway in the last 20 years, he said, you probably caught a glimpse of Insteel’s handiwork.

The wire product he makes is not unique, though, and he fears that if he has to charge commercial builders more, he will lose business to foreign competitors paying much less for their raw materials.