Per the 10th amendment, it’s a state issue so it can’t be done at the federal level unless you get a amendment, which won’t happen.
He didn’t say where in the US they need to be manufactured. Being that GM is owned now in part by the union, they are going to have to make some tough choices. Either pay the import tax or locate in Right-to-Work states because they will never be able to hit the price target if they are in compulsory union states.
Actually, let’s just use some the same logic that courts have just accepted to force gay marriage on us all. DOMA meant that the states had delegated the right to regulate marriage the Feds, so the Feds could decide what marriage meant.
The National Labor Relations Board, therefore, means that the states have delegated regulation of labor unions to the Feds.
Alternately: Labor unions are almost always inter-state organizations and the manufacturing sector unions make things intended for sale inter-state. Therefore, the Interstate Commerce clause can be used to regulate them.
Either way, problem solved.