“Scientific racism” is on the rise on the right. But it’s been lurking there for years.

“[Geert] Wilders understands that culture and demographics are our destiny,” Iowa Rep. Steve King tweeted earlier this month, referring to the far-right Dutch nationalist. “We can’t restore our civilization with somebody else’s babies.”

That’s a lot of racist theory to pack into 140 characters. The tweet evokes a fear of American decline caused both by genetics and culture, nature and nurture. Given the stark white nationalism on display in the message, it’s tempting to lump King in with his most vocal supporters — folks like alt-right leader Richard Spencer and Klansman David Duke — and dismiss his theories as part of the fringiest fringe.

But King’s theories about America’s cultural and demographic decay are not ideas carted in from Klan rallies or online alt-right message boards into a conservative political world that decisively rejects such notions. While his comments have drawn condemnation from some fellow congressional Republicans, they fit right in down the street at the White House, where top aides Steve Bannon and Stephen Miller serve as sentries against multiculturalism, shaping policies that have included the “Muslim ban” and...