Mario Cuomo is in many respects one of the most significant liberal politicians in the postwar era: a three-term governor of one the nation's largest states and an eloquent defender of the Democratic Party's progressive legacy during a period of conservative ascendancy. Yet in other respects he never lived up to his supporters' hopes. His gubernatorial record was spotty, and when he had the chance to seek the presidency, he equivocated, Hamlet-like,...