Campaigning for president in 1980, Ronald Reagan told stories of Cadillac-driving "welfare queens" and "strapping young bucks" buying T-bone steaks with food stamps. In trumpeting these tales of welfare run amok, Reagan never needed to mention race, because he was blowing a dog whistle: sending a message about racial minorities inaudible on one level, but clearly heard on another. In doing so, he tapped into a long political tradition that started...