When former President Donald J. Trump questioned Vice President Kamala Harris’s racial identity on Wednesday, he lifted a longstanding and false line of attack from the fringes of political discourse to the very center of a presidential campaign. For years, rivals and critics have lodged accusations that Ms. Harris shifts her personal identity to her political advantage, and that she is, in fact, not who she claims to be. Those attacks, based on falsehoods, misinformation and conspiratorial notions, have increased dramatically in the week and a half since she emerged as the Democrats’ all-but-certain standard-bearer. Just hours after President Biden...