Paul Ryan was a replacement theorist before most of us knew what that meant, likely including himself.
But then, the idea that the existing population of a place can decide what future populations look like is a longstanding tradition in America. You can see it in the genocide of indigenous populations, of course, and the literal domination of reproduction of black slaves. You can see it in eugenics, such as in the decision for Buck v. Bell when Oliver Wendell Holmes declared that "three generations of imbeciles is enough."
It's not happenstance that the same sort of people who endorsed turning indigenous children into "good Christians" also endorsed the sterilization of criminals and the mentally impaired, and now endorse forced childbirth while denying that aspiring citizens at the Southern border are even human.
That's replacement theory for you. That's what "Jews will not replace us" was all about-- the idea that white/Christian/"traditional"/conservatives have a right to exist in a country, and not just exist in it, but claim ownership of it.
Then the only course of action is to force more of those people into existence, while dehumanizing everyone else.