The first national immigration law in the United States, in 1790, limited naturalization to “free white persons of good character.” The belief that nonwhites were not full human beings entitled to citizenship was core to the institution of chattel slavery, which nearly destroyed the Union itself in a conflict that took more American lives than any other before or since. This tenet animated the violent overthrow of Reconstruction governments after the Civil War, and the century of Jim Crow segregation that followed. This conviction motivated the racist immigration-restriction laws at the turn of the century that helped inspire the Nazis and exacerbated the Holocaust. It shaped American policy from the Declaration of Independence to the New Deal, and it continues to do so. And it lives on today, in the minds of extremists who, like jihadists the world over, see themselves as the true guardians of their heritage.

Former President Barack Obama was mocked by the right for treading carefully around phrases such as Islamic extremism in an effort to avoid lending religious legitimacy to terrorist groups. But given the history of American military intervention in the Middle East, an American president can do little to erode the religious legitimacy of extremist figures who support terrorism; rather, such an effort rests on religious communities and leaders themselves—and contrary to the stereotype, condemnations of terrorism among Muslim religious leaders are frequent and voluminous. In the United States, that dynamic is reversed: American leaders can do a great deal to diminish the appeal of white nationalism, because unlike with jihadism, white Americans are the primary demographic targeted for radicalization.

“I believe the primary challenge posed by white nationalism is political, not law-enforcement related,” David Gomez, a former FBI counterterrorism supervisor, told me. “That is because, like fascism of the 1930s, white nationalism has begun to enter the mainstream political arena, which tends to make their extremism palatable to disenfranchised political minorities who fear the rise of ‘the other.’”

The spike in white-nationalist terrorism is emerging from the extremist fringe of the American right, giving conservatives a special responsibility to use their authority to deprive white nationalists of their claim to represent America’s authentic heritage. In the past, conservatives have actively opposed such efforts—in 2009, the Department of Homeland Security disbanded an entire unit devoted to tracking far-right terrorism after a backlash in which conservative writers suggested that mainstream conservatives were the ones being targeted.

This time around, some conservative writers and outlets are pressing for action. Others, including those who have the greatest influence with the president, have dismissed the threat of white nationalism while echoing its principles. The Fox News host Tucker Carlson, who has warned that immigration is “destroying America” and that “Latin American countries are changing election outcomes here by forcing demographic change on this country,” declares that white nationalism is a “hoax,” even as his programming echoes the very worldview held by white nationalists. That duality is clear to white nationalists themselves, who praise Carlson for amplifying their ideas even as he insists on their nonexistence.