Detroit, New York and Boston are very different cities with something in common: local politics are undergoing historic change. Last month, all three elected populist new mayors, who focused on unemployment and income inequality. Race wasn't an issue at all. In New York, Bill de Blasio campaigned against what he called the "two cities" presided over by his billionaire predecessor Michael Bloomberg. In Boston, the white union leader, Marty Walsh, got crucial support from minority candidates and won with a multi-racial coalition. In Detroit, Mike Duggan became the first white mayor elected in a black-majority city in 40 years. Big cities like these are America's economic engines, but two-thirds have yet to recover after the Great Recession. Washington is no longer leading the way, so voters are turning to City Hall. We look at the opportunities and the challenges lying ahead.