In his widely noted speech, President Obama said that "a dangerous and growing inequality and lack of upward mobility" is "the defining challenge of our time." This belief makes Mr. Obama unique: Unlike the other presidents since World War II, he places inequality above economic growth as the organizing principle of U.S. economic policy. The president's Dec. 4 speech, at an event hosted by the Center for American Progress, also stressed that increasing inequality is a "decades-long trend"—which carries with it the strong implication that the country needs to reverse the direction it has taken...