Which is not to say this new regulation is legally sound. As it stands , federal asylum law is unambiguous: Any person who is either “physically present” or “arrives” in the United States, no matter if said person arrived “at a designated port of arrival” or not, is entitled to apply for asylum. Desperate migrants, many of them fleeing violence or persecution with their families, cannot be expected to know the new rule Mr. Trump has imposed — only that asylum, as a matter of international law, may be within reach once they touch American soil. The president can’t, with the stroke of a pen, disregard people with credible asylum claims who don’t present themselves at a port of entry.

By issuing the ban and ensnaring those who don’t comply with it, the administration is clearly aiming to deny people largely from the Northern Triangle — Guatemala, El Salvador and Honduras — an opportunity to make their case for asylum and instead put them on a fast track to deportation. Many of them, in fact, have humanitarian reasons to flee their countries and seek shelter in the United States. Shutting off one avenue for them to do so and redirecting them to official border crossings will only lead to greater backlogs and discourage claims with real merit.

In anticipation of court challenges — the American Civil Liberties Union on Friday announced the first lawsuit over the proclamation — the Department of Homeland Security has published a tipsheet trying to soften the blow. There we’re told that Mr. Trump’s proclamation is not just lawful but also constitutional, relying on the same misguided Supreme Court case that eventually blessed Mr. Trump’s entry ban on Muslim travelers.

Like Mr. Trump’s wasteful, needless deployment of thousands of active-duty troops to the border, the asylum ban works more or less like an imaginary wall: a substitute for the physical wall that even unified control of Congress could never give him. And that a Democratic-controlled House never will.

Mr. Trump may have left some with the impression that, once the midterms were over, he had no more use for the caravan of Central American migrants that he had fantasized about to frighten his supporters to the polls. But like the Muslim ban, which was also gestated in a political contest, the one that elected him president, Friday’s asylum ban demonstrates that Mr. Trump’s nativist impulses are guides not just to his campaigns but also to his governing.

He creates nonexistent threats, generates manufactured fears and prepares exaggerated responses, all leading to an erosion of the rule of law, to feed more fears.