Last week's Pennsylvania primary demonstrated that Barack Obama is not unbeatable. This might sound a strange way to put it. Hasn't it always been true that Obama is beatable? Well, consider an alternate reality in which Obama had won Pennsylvania. His people certainly thought long and deeply about this alternate reality--why else spend a staggering $12 million on one state's primary? Hillary Clinton would have dropped out. Obama would have shown that he can win white working-class votes in a big, diverse, populous state. Way back after the Iowa caucuses, he playfully observed that everywhere he goes becomes Obama country....