The goal here is not to sway listeners. Regular listeners are already on board, as others have pointed out. The goal is to motivate the faithful to become active. To talk to friends, family and co-workers. To attend rallies. And, of course, to donate money. I agree completely with Hugh. Radio is definitely underappreciated as a political messaging vehicle.
On a positive note for radio, I teach radio and TV at a DC area community college, and enrollment has been up the last few years in the radio curriculum.