Replacement theory -- the racist ideology that claims elites are abetting immigrants to disempower or eliminate native white people -- has been around in one form or another for a long time. The current iteration has gone mainstream, leading to widespread condemnation of some Republican politicians and conservative commentators who have embraced the theory's central premises. Fear and suspicion of foreigners underpins nativism, and America's first nativist movement took hold in the 1850s. Who were the Know Nothings? They weren't around for long but they left their mark.