Many on the center-left responded angrily to the essay, asserting that any liberal or progressive program worthy of the name needs to treat citizens and non-citizens identically — a position that implies the moral illegitimacy of borders. This, in turn, inspired The New York Times’ Ross Douthat to suggest, quite accurately, that “liberalism’s current relationship to open borders is asymptotic: Not for it, but for every step toward it.”

As David Frum notes in his own more recent and very powerful essay for The Atlantic, this tendency has been “turbocharged” by the Trump administration’s “zero tolerance” policy along the southern border, and especially by its (now suspended) policy of separating parents and children when they are apprehended entering the country illegally. In reaction to the administration’s brutality, liberals have moved further than ever in the direction of embracing the view that enforcing the distinction between those who enter the country legally and illegally, and punishing or deporting those in the latter category, is morally suspect. From there it is one small step to declaring and defending a right to the free movement of persons across borders.

Without even taking that last small step, the center-left’s sweeping declarations of universal humanitarianism play into Trump’s xenophobic hands by lending plausibility to his claim that he’s defending the good of the country by combating “extremist open border Democrats.”