On September 7, 1964, a 60-second TV ad changed American politics forever. A 3-year-old girl in a simple dress counted as she plucked daisy petals in a sun-dappled field. Her words were supplanted by a mission-control countdown followed by a massive nuclear blast in a classic mushroom shape. The message was clear if only implicit: Presidential candidate Barry Goldwater was a genocidal maniac who threatened the world’s future. Two months later, President Lyndon Johnson won easily, and the emotional political …