So Perlstein is talking about something real. The trouble is the polemical use he wants to make of this reality.

Perhaps a counter-example will clarify the problem.

When conservatives want to rebut accusations of racism, they often deploy the talking point: The Ku Klux Klan—meaning the Klan of the 1860s and 1870s—was founded by Democrats. Rand Paul built a speech at Howard University on the claim; Dinesh D’Souza produced an entire documentary to illustrate it.

The “Democrats founded the Klan” talking point is, of course, literally true. But it’s not deployed in the service of truth. It’s designed as an excuse and an attack, not an explanation.

And so it is with Perlstein’s trope about Trump and the Klan of the 1920s. It’s an important question whether the success of Donald Trump—and the rise of similar authoritarian populists in France and elsewhere in Europe—is the recrudescence of something old or the appearance of something new. The more worried you are about Trump and Trumpism, the more urgent this question should be to you—because only the correct answer will lead to a wise response.

Somehow an explanation of Donald Trump’s political success has to incorporate the fact that Trump won a higher share of the Latino vote and black vote in the presidential election of 2016 than Mitt Romney did in 2012- and that Hillary Clinton did worse with white voters than Barack Obama had done. There’s little question that inter-ethnic animosity explains much about the 2016 vote. But it seems simply blind to pretend that these animosities straightforwardly carry over into 2016 the attitudes and patterns of almost a century ago. The study of continuity and discontinuity is literally the historian’s job. When the historian instead baldly asserts that anything he dislikes is the same as everything else he dislikes—well, then the job of the historian is abdicated.

At the foundation of Perlstein’s intellectual dilemma is the following paradox. Perlstein strongly identifies as a political progressive. Fine for him, it’s a free country. Contemporary progressivism values both cultural cosmopolitanism and also economic egalitarianism; both diversity and equality. People strongly committed to progressive politics can readily take for granted that their two deepest commitments inherently and necessarily belong together. But that’s really not true.

Through most of the history of ideas, the great champions of equality have been xenophobes—not out of bigotry, but because they believed that only a tightly bonded society could suppress the concentration of wealth in the hands of a few. Let Jean-Jacques Rousseau speak for a tradition that commenced with the Greeks and continued to the Jacobins and beyond: The best leaders determine “that his people should never be absorbed by other peoples” and therefore “devised for them customs and practices that should could not be blended into those of other nations.” Ideally: “Each fraternal bond … among the individual members of his republic became a further barrier, separating them from their neighbors and keeping them from becoming one with those neighbors,” he wrote in his Considerations on the Government of Poland. As for more cosmopolitan societies, Rousseau had this disdainful comment in The Social Contract: “The people has less affection for the homeland which is like the whole world in its eyes, and for its fellow citizens, most of whom are foreigners to it.”