Nothing. The way you stop outsourcing labor to cheaper locations is by taxing the product on import to neutralize the difference. If the US did that since the beginning of the global trade, the US would still be manufacturing everything that was outsourced to China. The issue with Trump’s tariffs is that they’re not targeted to just things the US manufactures or things that can reasonably restart manufacturing. These tariffs for example are helping auto manufacturing but make aluminum and parts needed for that manufacturing more expensive. So they’re nonsensical and could actually cost jobs.

Entirely separately, merely protecting jobs from outsourcing isn’t enough because, if labor unions are busted and domestic wages are stagnant.

Well the context is, he supports tariffs that protect American factory jobs. He supports investing in worker protections and domestic factories. He’s basically saying that when American factories exist for a product, products made elsewhere with cheaper foreign labor should have tariffs on them to raise the price of those foreign goods to protect domestic workers. That is actually a reasonable position.

What is not a reasonable position is imposing tariffs when we have no domestic production while simultaneously destroying American worker protections to ensure that we continue to have no domestic production and that workers have even less than they did before.