A key precept of the Trumpian ethos is that immigration is a threat not only to American culture but to its prosperity. President Donald Trump and his allies have long used the language of the “great replacement” conspiracy theory, the racist belief that white people and Western culture are being “replaced” by nonwhite immigrants.In a similar vein, the Trump administration has warned of another kind of “replacement”: immigrants taking jobs that would otherwise be filled by natural-born American...