If you wanted to ensure the eventual triumph of immigration restrictionism in the United States, you couldn't devise a surer path to that goal than getting the Democratic Party to explicitly embrace a policy of de facto open borders.

Unfortunately, this is precisely where the liberal reaction to President Trump's viciously harsh immigration policies is headed.

The trend has been underway for years. Immigrant and refugee activists have long made the case, on humanitarian grounds, that pretty much anyone seeking asylum should be granted it. In recent years, those arguments have gained considerable ground on the left and even on the center-left. Peter Beinart noted as much in an important piece for The Atlantic last summer that expressed concern about its possible negative electoral consequences for Democrats.

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."

Those who doubt the reality of this dynamic need only look across the Atlantic to Europe. Populist and nationalist parties have been on the rise for the better part of a decade, responding at first to structural economic problems and frustrations with the bureaucratic burdens of the European Union. But they received a potent infusion of electoral energy only after Angela Merkel unilaterally decided in 2015 to open Europe's borders to a flood of refugees from the Middle East, and especially from Syria's bloody and intractable civil war.

Suddenly the populists and nationalists had an injustice to stoke, a cause to champion, an anxiety to provoke, and a foil to use for rallying supporters. Suddenly what would have been dismissed by most voters just a year or so earlier as an extremist conspiracy theory began to sound reasonable, credible, wise. Overnight, immigration restrictionism had become a sensible, mainstream position.

Three years later, we've had the Brexit vote in the U.K. and the coming to power or electoral strengthening of populists and nationalists in Hungary, Austria, Poland, the Czech Republic, Germany, and Italy, with Sweden poised to become the next domino to fall in September elections. Populists and nationalists have even made significant headway in gaining power within the European Parliament itself. (Currently, 151 out of 751 members of the legislative body are "openly critical or hostile toward the EU.")

Europe has a much rockier history than the United States of responding to immigration pressures. But that doesn't mean we're immune to the same dynamic. Every political community draws a line somewhere, and the placement of that line shifts over time. From the closing decades of the 19th century down to the early 1920s, the U.S. admitted (with most people welcoming) many millions of immigrants. That changed in 1924, when the doors were mostly closed. Forty years later, they were opened once again, albeit using a different set of criteria for admission. Since then, immigration has surged.

Request for entry both here and in Europe is not likely to diminish over the coming years, with economic disparities between the developed and developing worlds remaining stark, and the number of refugees and other migrants (many through forced displacement) at record highs and likely to remain there.

In such a situation, it's crucially important for voters to know that, wherever we ultimately choose to set the annual rate of immigration and whatever admissions criteria we use, we will enforce it, by taking firm control of the border and continuing to uphold the politically crucial distinction between who is and who is not a citizen.

The alternative, as Frum notes in his essay, isn't a world of placeless humanity in which individuals move freely across porous (and ultimately pointless) national borders. It's a world in which sub-national tribes step in to assert the control that national authorities have irresponsibly relinquished.

The human longing for communal solidarity — and its corollary, the love of and privileging of one's own above others — cannot simply be wished or legislated away. No matter how much moral discomfort it causes.