The same cannot be said of white nationalists, who today are on the march from Charlottesville, Va., to Dresden, Germany. As an ideology, white nationalism poses a significantly greater threat to Western democracies; its proponents and sympathizers have proved, historically and recently, that they can win a sizable share of the vote — as they did this year in France, Germany and the Netherlands — and even win power, as they have in the United States.

Far-right leaders are correct that immigration creates problems; what they miss is that they are the primary problem. The greatest threat to liberal democracies does not come from immigrants and refugees but from the backlash against them by those on the inside who are exploiting fear of outsiders to chip away at the values and institutions that make our societies liberal.

Anti-Semitic and xenophobic movements did not disappear from Europe after the liberation of Auschwitz, just as white supremacist groups have lurked beneath the surface of American politics ever since the Emancipation Proclamation. What has changed is that these groups have now been stirred from their slumber by savvy politicians seeking to stoke anger toward immigrants, refugees and racial minorities for their own benefit. Leaders from Donald Trump to France’s Marine Le Pen have validated the worldview of these groups, implicitly or explicitly encouraging them to promote their hateful opinions openly. As a result, ideas that were once marginal have now gone mainstream.

The trend is unmistakable. Hungary’s ruling party has plastered anti-Semitic ads on bus stops and billboards; an overtly neo-Nazi movement won 7 percent of the vote in Greece’s 2015 election; Germany’s upstart far-right party, which includes a popular member who criticized Berlin’s Holocaust memorial as “a monument of shame,” won 13 percent in last month’s election.

In France and Denmark, populist leaders have gone to great pains to shed the right’s crudest baggage and rebrand themselves in a way that appeals to Jews, women and gay people by depicting Muslims as the primary threat to all three groups. But their core goal remains the same: to close the borders and expel unwanted foreigners.

Cultural and demographic anxiety about dwindling native populations and rapidly increasing immigrant ones lies at the heart of these parties’ ideologies. In America, Representative Steve King, Republican of Iowa, worries about the impossibility of restoring “our civilization with somebody else’s babies.” In Europe, the right frets about who’s having the new German or Danish babies and the fact that it’s not white Germans or Danes — a social Darwinist dread popularized by the German writer Thilo Sarrazin, whose best-selling 2010 book, “Germany Abolishes Itself,” warned that barely literate Muslims were poised to replace the supposedly more intelligent German race.

The leader of the Netherlands’ newest far-right party fears that Europe will not exist “as a predominantly white-skinned, Christian or post-Christian, Roman-law-based kind of society” a few decades from now. “If I go to a museum, and I look at these portraits, they are essentially people like me that I can see. In 50 years it won’t be,” he worries.

France, more than any other country, has been the source of these ideas.

***

IN FEBRUARY 2016, French right-wing groups descended on the town of Calais, protesting a huge informal refugee camp there known as the “Jungle.” Members of the German anti-Islam group Pegida (the name is short for the German words for Patriotic Europeans Against the Islamization of the West) came, too. Demonstrators clashed with local policemen, and a decorated French paratrooper marching alongside them was arrested. A van marked with the logo of a medical charity aiding Jungle residents was set on fire one evening, and the group’s volunteers had their tires slashed.

A few months later, I met the leaders of a local anti-immigration group called Retake Calais. When I asked if they wanted to see the migrants leave town, they lamented that closing the camp — which has since been bulldozed — wouldn’t help. “They’re sending them to all the little villages in France,” one of them told me. “In two years the villages will be dead.”

“It’s the great replacement,” his friend added, echoing the title of a 2010 book by the French writer Renaud Camus, which paints a dark picture of demographic conquest in the West. “They want to replace us.”

As Mr. Camus explains in the book: “You have a people and then, in an instant, in one generation, you have in its place one or several other peoples.” He finds it scandalous that “a veiled woman speaking our language badly, completely ignorant of our culture” is legally considered as French as “an indigenous Frenchman passionate for Romanesque churches, and the verbal and syntactic subtleties of Montaigne and Rousseau.” In Mr. Camus’s eyes, groups like Pegida are heroic. He praises the group as a “liberation front” that is battling “a colonial conquest in progress” where white Europeans are “the colonized indigenous people.”

Ms. Le Pen, the leader of France’s far-right National Front party, has a similar fear, and she sees birthright citizenship as the vehicle for replacement. Although she doesn’t use the term favored by many Republicans in the United States (“anchor babies”), she insists, as she told me in an interview last May, that “we must stop creating automatic French citizens.”

This argument has a long pedigree. It can be traced back to the Dreyfus Affair, when the virulently anti-Semitic writer Maurice Barrès warned that immigrants wanted to impose their way of life on France and that it would spell the “ruin of our fatherland.” “They are in contradiction to our civilization,” Barrès wrote in 1900. He saw French identity as rooted purely in his bloodline, declaring, “I defend my cemetery.”

Today’s version of the argument is: if you have foreign blood and don’t behave appropriately, then you don’t get a passport.

The notion of a Great Replacement has crossed the Atlantic and found an eager audience among groups who have long espoused similar white supremacist ideas. The Dutch far-right leader Geert Wilders warned in 2015 of “masses of young men in their 20s with beards singing ‘Allahu akbar’ across Europe.” He labeled their presence “an invasion that threatens our prosperity, our security, our culture and identity.”

A year later, Mr. Wilders attended the Republican national convention, where he headlined an L.G.B.T. pro-Trump event along with the anti-Islam activist Pamela Geller and the alt-right wunderkind Milo Yiannopoulos. Before he began his talk in front of a wall featuring photos of barechested men, “Make America Great Again” hats and a “Don’t Tread on Me” flag, Mr. Wilders was introduced as “the hope for Western civilization.”

Calais and Charlottesville may be nearly 4,000 miles apart, but the ideas motivating far-right activists in both places are the same. When white nationalists descended on Charlottesville in August, the crowd chanted “Jews will not replace us” and “you will not replace us” before one of its members allegedly killed a woman with his car and others beat a black man; last week, they returned bearing torches and chanting similar slogans.

Just as Mr. Trump has plenty to say about Islamic State attacks but generally has no comment about hate crimes against Indians, blacks and Muslims, the European far-right is quick to denounce any violent act committed by a Muslim but rarely feels compelled to forcefully condemn attacks on mosques or neo-Nazis marching near synagogues on Yom Kippur.

Doing so might alienate their base. Alexander Gauland, a co-leader of the newest party in the German Parliament, is adamant that his Alternative for Germany is “not the parliamentary arm of Pegida,” although he did acknowledge in an interview that “a lot of people who march with Pegida in Dresden are people who could be members, or friends, or voters” for the party. Like Mr. Trump, Mr. Gauland and Ms. Le Pen would never admit to being white nationalists, but they are more than happy to dog-whistle to them and accept their support.

Those who worry that a godless Europe and an immigration-friendly America are no match for Islamic extremists have ignored an even greater threat: white nationalists.

Their ideology is especially dangerous because they present themselves as natives valiantly defending the homeland. Because they look and sound like most of their co-citizens, they garner sympathy from the majority in ways that Islamists never could. White nationalism is in many ways a mirror image of radical Islamism. Both share a nostalgic obsession with a purist form of identity: for one, a medieval Islamic state; for the other, a white nation unpolluted by immigrant blood.

If the influence of white nationalists continues to grow, they will eventually seek to trample the rights of immigrants and minorities and dismiss courts and constitutions as anti-democratic because they don’t reflect the supposed preferences of “the people.” Their rise threatens to transform countries that we once thought of as icons of liberalism into democracies only in name.