Reihan Salam explains:

Not only did Obama not expand beyond his core constituencies–as always, he was crushed among Catholics, an atypically big slice of Pennsylvania’s Democratic electorate, and white working-class voters–he lost
ground with affluent professionals, the group that has powered his historic fundraising success, with weekly churchgoers, and with the moderates who have until recently seen him as one of their own. He lost Greater Pittsburgh and the Philadelphia suburbs by wide margins, and he also lost the northeastern part of the state by a whopping 66 to 34 percent. In a new Brookings study of Pennsylvania’s political demographics, William Frey and Ruy Teixeira identify this region, centered on Allentown, as key to the state’s political future. If Pennsylvania’s Northeast keeps trending Democratic, the state will become solidly blue. But if a Republican candidate can hold the line or make some modest gains with the region’s white working class voters, the picture looks very different. And as it turns out, the GOP may have a candidate who can do just that in John McCain. As Hillary Clinton’s campaign slow-marches to its unhappy end, she is offering lessons not only for how McCain can defeat Obama–she is pointing towards a possible bright future for the Republican brand. She’s probably not thrilled about that. But before we get ahead of ourselves, it’s worth considering the scale of the obstacles Republicans face.

Note that Salam doesn’t agree that McCain is by any means a lock, but I think that this paragraph explains Obama’s big problem in winning in November. Hillary! has a different set.