Shortly after Republicans swept last November to a historic victory in which Sarah Palin was credited with playing a central role, the former Alaska governor pulled aside her close aide, Rebecca Mansour, to discuss a hush-hush assignment: Reach out to conservative filmmaker Stephen K. Bannon with a request. Ask him if he would make a series of videos extolling Palin's governorship and laying to rest lingering questions about her controversial decision to resign from office with a year-and-a-half left in her first term. It was this abdication, Palin knew, that had made her damaged goods in the eyes of some...