President Trump is taking flak from all sides for ending his predecessor’s Deferred Action for Childhood Arrivals (DACA) policy, thus putting some 800,000 young immigrants—so-called Dreamers—in legal limbo. Though the President and Barack Obama share responsibility for instigating the crisis, Mr. Trump and Congress now have an obligation to fix it and spare these productive young adults from harm they don’t deserve.

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Mr. Trump was at his worst during the campaign when he assailed DACA as an “unconstitutional executive amnesty,”...