Bernie Sanders is surging. What has changed? It cannot be the candidate, because, rightly or wrongly, voters perceive Mr. Sanders as always being his same sincere, authentic self. One thought is that, given how the Democratic field has narrowed, he might in the end be the man to beat President Trump, since he appeals to many of the angry citizens who proved decisive in 2016 — an anti-establishment message vigorously pushed by the Sanders campaign itself.

The obvious implication is that Mr. Sanders and Mr. Trump must have things in common: It has been claimed that both are populists, both divisive, both ready to break norms, both attack the news media, both have cult followings — the list could go on. Today, this false equation is being pushed by pundits on the right and, until now less loudly, by liberal centrists. But it is high time to retire the horseshoe theory according to which extremes of right and left must always meet somewhere, such that the only salvation from “political sociopaths” lies with the center.

Conventional wisdom has it that populists criticize elites. In recent years, with the rise of populists around the globe, there is also the suspicion that they pose a systematic threat to liberal democracy as such. But it is wrong simply to equate populism with being “anti-establishment.” After all, any old civics textbook will say that keeping a critical eye on the powerful is actually a hallmark of being a good democratic citizen, and nothing particularly populist.

Figures like Mr. Trump, Prime Minister Viktor Orban of Hungary or President Recep Tayyip Erdogan of Turkey do indeed criticize elites. But more important, they claim that they, and only they, represent what they often call the “real people.” As a result, they do not just attack other contenders for power as fundamentally illegitimate, they also suggest that all citizens who do not support them are not properly part of the people at all. Mr. Trump does not simply try to refute opposition to his policies, as any normal politician would do; he attacks his critics as “un-American.” Mr. Erdogan says about himself and his political party, “we are the people.” Right-wing populists always wage culture wars with a view to excluding some citizens, symbolically or through actual disenfranchisement. While they incessantly talk about unifying the people, dividing them is actually the very basis of their political business model.