It’s important to realize that Trumpian protectionism wasn’t a response to a groundswell of public opinion. As best as I can tell from the endless series of interviews with white guys in diners — who are, we all know, the only Americans who matter — these voters are driven more by animosity toward immigrants and the sense that snooty liberals look down on them than by trade policy.

And public opinion seems to have become far less protectionist even as Trump has raised tariffs, with the percentage of Americans saying that free trade agreements are a good thing as high as it’s ever been.

So Trump’s trade war is losing, not gaining, support. And one recent analysis finds that it was a factor hurting Republicans in the 2018 midterm elections, accounting for a significant number of lost congressional seats.

Nevertheless, Trump persists. Why?

One answer is that Trump has long had a fixation on the idea that tariffs are the answer to America’s problems, and he’s not the kind of man who reconsiders his prejudices in the light of evidence. But there’s also something else: U.S. trade law offers Trump more freedom of action — more ability to do whatever he wants — than any other policy area.

The basic story is that long ago — in fact, in the aftermath of the disastrous Smoot-Hawley tariff of 1930 — Congress deliberately limited its own role in trade policy. Instead, it gave the president the power to negotiate trade deals with other countries, which would then face up-or-down votes without amendments.

It was always clear, however, that this system needed some flexibility to respond to events. So the executive branch was given the power to impose temporary tariffs under certain conditions: import surges, threats to national security, unfair practices by foreign governments. The idea was that nonpartisan experts would determine whether and when these conditions existed, and the president would then decide whether to act.