And so an old rallying cry is getting refurbished for a new generation.

“The way that emotion gets engaged in the right wing today is almost always around questions of fertility,” said Paola Bacchetta, a professor at the University of California, Berkeley, who serves on the board of the Center for Right-Wing Studies. “It’s about their anxieties about their male others. They fear that they will overproduce them and eliminate them.”

Though these fears fester in online message boards, they are spreading to more mainstream right-wing conversations. The Fox News host Tucker Carlson, for one, has engaged in some of these conversations. In January, he opened his show with a segment he said was “the biggest issue facing this country going forward,” bigger than wars and G.D.P.: the collapse of families. The major cause of that collapse, he said, was that some women now out-earn some men.

(Mr. Carlson’s discussion of the topic attracted fierce criticism, something he addressed on his show the next night. “This is why important science is no longer being conducted,” he said of the response. “It’s why art isn’t being made and comedy is dying.”)

The birthrate panic has been bubbling back up for some time. In a 2012 book by the French philosopher Renaud Camus, he argued that all Western countries were reckoning with erasure by birthrate. That has helped fuel nativist campaigns like the one by the Dutch politician Geert Wilders. It became the animating philosophy in the Charlottesville, Va., attack. And Representative Steve King, the Iowa Republican, tweeted in 2017, “We can’t restore our civilization with somebody else’s babies.”

Many of these groups have seized on statistics showing a slowing birthrate in some Western countries. In the United States, the total fertility rate is now about 1,765.5 births per 1,000 women, according to the Centers for Disease Control and Prevention, which is below the level necessary to keep the population stable without immigration.