Three shootings, three different targets — but all linked by one sprawling, ever-mutating belief now commonly known as replacement theory. At the extremes of American life, replacement theory — the notion that Western elites, sometimes manipulated by Jews, want to “replace” and disempower white Americans — has become an engine of racist terror, helping inspire a wave of mass shootings in recent years and fueling the 2017 right-wing rally in Charlottesville, Va., that erupted in violence.

But replacement theory, once confined to the digital fever swamps of Reddit message boards and semi-obscure white nationalist sites, has gone mainstream. In sometimes more muted forms, the fear it crystallizes — of a future America in which white people are no longer the numerical majority — has become a potent force in conservative media and politics, where the theory has been borrowed and remixed to attract audiences, retweets and small-dollar donations…

No public figure has promoted replacement theory more loudly or relentlessly than the Fox host Tucker Carlson, who has made elite-led demographic change a central theme of his show since joining Fox’s prime-time lineup in 2016. A Times investigation published this month showed that in more than 400 episodes of his show, Mr. Carlson has amplified the notion that Democratic politicians and other assorted elites want to force demographic change through immigration, and his producers sometimes scoured his show’s raw material from the same dark corners of the internet that the Buffalo suspect did.