Later, during his second year in the U.S. Senate, Obama called Clark, among others, to discuss whether it made sense for him to mount a bid for the Presidency. Clark, 62, is one of Chicago's elder statesmen and chief of ComEd, a subsidiary of energy giant Exelon (EXC) and the largest electric utility in Illinois, serving nearly 4 million customers in Chicago and Northern Illinois. He didn't mince words: "Your window of opportunity is now," Clark recalls saying. "Go do it."