Whatever becomes of former President Donald Trump in our politics, courts and culture, Trumpism will linger and try to metastasize. Trumpism raises big questions not only about what we expect from a president but also what America is and wants to be. Trump’s ugliest legacy is how he seemingly reenergized the racism that has existed in America since its founding. The manifestations include a longtime conspiracy theory called the “great replacement.” White supremacists are injecting it into the political mainstream, especially within the Republican Party, particularly to fuel immigration fears. What is the “great replacement” theory? An expert witness, Elizabeth...