Unintended harm to American companies is a recurring problem with tariffs, even those meant to protect American jobs from competition that our government deems unfair. After Bush imposed steel tariffs, steel-consuming industries pointed out that they employed far more Americans than the steel industry itself, and argued  that the net effect of the policy on jobs was negative.
Anti-dumping laws, which put tariffs on foreign imports that are supposedly being sold at too low a price, usually target intermediate goods and therefore make the downstream American producers that use them less competitive . Daniel Ikenson, a trade-policy analyst at the Cato Institute, notes that the government, perversely, is forbidden by law from considering the impact of tariffs on these producers before levying the tariffs.
Nafta has also helped U.S. industry stay globally competitive. Many companies have moved plants to Mexico, but they often supply parts for U.S. finished goods. An April 2015 Congressional Research Service study credits Nafta “with helping U.S. manufacturing industries, especially the U.S. auto industry, become more globally competitive through the development of supply chains.” By moving some low-wage jobs to Mexico, car makers have been able to maintain U.S. production.