FDR brought with him to office the old Democratic favoritism toward free trade, but also decidedly Wilsonian views on the relations between president and Congress. He encouraged Congress to transfer authority on trade matters (as well as most regulatory matters!) to him. This time, the legislature agreed. It was as if Congress threw up its hands in exasperation and said to the president, “We cannot handle our authority responsibly. Please take it off our hands, for we will screw things up and lose reelection.”
So more and more over the past 80 years, authority over tariffs, as well as over all manner of properly legislative functions, has migrated to the executive branch, away from the legislative — even in instances (such as this aluminum-and-steel case) where there is no compelling or immediate foreign-policy mandate. Trump’s move is purely a play for aggrieved industrial workers, who should, in the constitutional schema, look to Congress and not the president for redress of their grievances.
And this is exactly the problem with our government in 2018. Nobody looks to Congress for redress of grievances anymore, for it would be foolish to do so!