Donald Trump is leaning into a nativist, anti-immigrant message in the final stage of his third presidential campaign, advancing a closing argument centered on fearmongering, falsehoods and stereotypes about migrants as polls show his edge on economic issues fading. In recent days, the former president has suggested that “bad genes” are to blame for people in the country illegally who have committed murders, reprised his warnings about a migrant “invasion” and suggested Vice President Kamala Harris’s handling of border issues shows she is “mentally impaired.” He will campaign in Aurora, Colo., on Friday, after promoting false claims about Venezuelan gangs...