The biggest economic risk of a Donald Trump Presidency has always been that his trade obsessions would swamp the benefits of tax reform and deregulation. For two years he has kept his worst protectionist impulses mostly in check, but as he seeks a second term we are now seeing Tariff Man unchained. Where he stops nobody knows, which is bad for the economy and perhaps his own re-election.

Mr. Trump’s trade talks with China have broken down and both countries are escalating their rhetoric. The U.S. is blocking Huawei from buying U.S. products, while China is threatening to block the export of rare-earth minerals that are crucial to many U.S. industries. A trade deal that looked imminent weeks ago now looks months away, if it happens at all. Mr. Trump’s threat of 25% tariffs on foreign autos also hangs over Europe and Japan.

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Then out of the blue late Thursday Mr. Trump announced escalating tariffs up to 25% on U.S. imports from Mexico for alleged sins on immigration. The first 5% levy would hit June 10, with 5% increments through Oct. 1. “Mexico must step up and help solve this problem,” Mr. Trump declared in a statement charged with political rhetoric. “For years, Mexico has not treated us fairly—but we are now asserting our rights as a sovereign Nation.”

The first problem here is that Mr. Trump is blaming Mexico for a mess it can’t solve. The real cause of the recent border chaos is the lure of U.S. asylum policy. Migrants from Central America know that if they cross the border illegally with children they can’t be detained for more than 20 days. They are then released into the U.S. where they can work for years until their asylum hearing date, and most never show up.

Mr. Trump is right that Democrats refuse to help, but then why change the subject to Mexico? Our southern neighbor is already helping by agreeing to hold asylum seekers inside Mexico. Perhaps it could better control its border with Guatemala, but the caravans north are often led by gangs that know how to bribe or avoid police. Blaming Mexico is a distraction that lets Democrats off the hook.