A dozen years out of Harvard, investment banker Bill Middendorf's salary hit $250,000 a year; another dozen years, with his own firm and a seat on the New York Stock Exchange, his income was well into seven figures. But he was restive. "I had learned how to make money," he writes. "I wanted to learn how to make a difference." Thus, he became actively involved in politics, first at the local level and then with the presidential campaign of Senator...