Everyone is thinking it.

But no one wants to talk about it.

Éric Zemmour, the far-right media pundit who is now running for president of France, is Jewish. I would not want this fact to discourage anyone from asking what the Zemmour phenomenon reveals about France’s exhausted political system; or about the collapse of the Marine Le Pen party; or about the coming takeover on the moderate right that will likely make it possible for Zemmour to raid the war chest of Les Républicains party in 2021.

People should also reflect on the dark ideas this pugilistic candidate is hatching, the poisons he is serving up, and the shrunken, pitiful version of France he is promoting when he declares that we have “no business” getting involved in the fate of Afghan women, or that we “will never know” the truth about the Dreyfus affair, or that we should disapprove of the innocent souls murdered by Mohammed Merah in 2012, whose parents “buried their bones” in Jerusalem.

Against each of these grotesque offenses I will speak out.

I’ve had the occasion to debate Zemmour when he was still, like Mussolini, just a small, self-obsessed journalist. I know his tricks well enough to put him in his place when the time comes … unless his bubble pops first. But, today, I pose another question: What does Zemmour mean for the Jews?

Optimists will insist that it is the voter base of Marine Le Pen, not that of center-right candidate Valérie Pécresse, that Mr. Zemmour has begun to splinter, which is not a bad thing. The wryly inclined will appreciate the historical irony—if it is not a trick or a trap—of the old antisemitic far right championing a type of man who was not exactly their sort. Perhaps there will even be lovers of the Romanesque marveling that such a tale could only have been cooked up by a Philip Roth (Operation Shylock) or a Romain Gary who, in The Dance of Genghis Cohn, imagined an old Nazi inhabited, voiced, by a little demon—a dybbuk who was a Jewish survivor of the Holocaust.

Pessimists, on the other hand, will see Zemmour inflaming the worst obsessions of the far right, and will fear that the association of his Jewishness with those obsessions will ignite the antisemitism of the far left that is waiting in the wings.

Someday, no doubt, historians will also see in this affair an extreme case of the mechanism described by Hannah Arendt: “Israelites” so hopelessly taken with Frenchness that their Jewishness becomes, as for Marcel Proust’s Bloch, a source of self-hate. Or German Jews donning the spiked helmets they had saved from 1914 when, in 1933, the Nazis arrived to prepare them for the camps. So, why not a Zemmour—whose parents were, like mine, stripped of their citizenship by Vichy—crying from the rooftops that Pétain protected them?

But the most burning question lies elsewhere.

I watch him rushing to embrace the worst rhetoric of Maurice Barrès and Charles Maurras as if he wanted to smash the synagogues on the martyred pediment of Notre-Dame. I see him trampling on everything in the French Jewish legacy that pertains to responsibility for others, or the noble effort to embrace strangers, love thy neighbor, and offer hospitality toward migrants.

In this transgression there is something that chills the blood.

I said as much five years ago to American Jews tempted by Trumpism: To make a pact with that, to follow a bad shepherd who respects only power, money, and the tinsel of his own palaces, could be likened to suicide. Now French Jews are facing the same choice.

And I say it again today to French Jews tempted to identify with the sinister simplifications of Éric Zemmour, who, they believe, will take their side against those who attack schools and synagogues, and who see the failure of the state to deliver justice in the case of Sarah Halimi and others as a threat—not unreasonably—to their own lives: The enemy of your enemy can be your enemy, too. His nationalistic and racist hubris; his cruelty; his renunciation of Jewish generosity, vulnerability, humanism, and sense of otherness; his ignorance of the real knowledge, written in blood in family memories, that argues for wariness in the face of history’s whirlwinds and its acid jets of persecution—these are an insult to the Jewish name that all Jews carry within them, unless and until they explicitly throw it overboard.

Mr. Zemmour is certainly not the first person to create the impression that one can be both a Jew and a populist extremist. And fortunately, there will always be Jews assertive enough to object that Paul Claudel himself would not have wanted anyone to have to choose between Claudel and the Talmud. But this is not a contest of ideas or aesthetics in which we welcome the widest variety of challengers in order to sharpen and strengthen our own views. It is a contest for raw political power, which in turn will directly affect the fate of Jews living in France and beyond.

The size and force of the wave that Zemmour is riding should not be underestimated. The idea that in the pursuit of power he will desecrate his own name and the name of our people—and in so doing, become the instrument of forces against which Jewish hopefulness has fought for millennia—is unbearably obscene.

A peril haunts the metaphysical house that has sheltered a share of our sense of the human. It was France, following in the footsteps of the American Revolution, that first recognized the Jew of Europe as a citizen, expanding the circle of humanity outward to include the other in a gesture that would be repeated throughout the next two centuries to create the modern liberal model of social solidarity that characterizes and legitimates Western democracies. How bitter the historical irony if the instrument of that order’s undoing turns out to be a French Jew.