President Trump tweeted “big news coming soon for our great American Autoworkers” ahead of his Administration’s announcement last week that it will seek new tariffs on imported autos and auto parts. If he had more Twitter characters, perhaps Mr. Trump would have added, “Tough luck for the rest of you.”

Just kidding, but that would have been accurate. Mr. Trump is boasting of potential gains for one narrow constituency from his latest protectionist threat. But he’s ignoring the forgotten Americans who will lose their jobs when...