Only 100 people were listening, but the first broadcast from a licensed radio station occurred at 8 p.m. on Nov. 2, 1920. It was Pittsburgh’s KDKA, and the station was broadcasting the results of that year’s presidential election. When the man responsible, Frank Conrad, flipped the switch for the first time, he couldn’t have envisioned just how profoundly broadcast media would transform political life. For centuries, people had read politicians’ words. But radio made it possible...