In context, it was a reasonable thing to say, but perhaps poorly expressed, since Obama kind of implied that people were voting these other issues just because they were bitter. Hillary could have gotten some mileage out of it against a lesser candidate, but she's up against the Michael Jordan of Democratic politics. The guy just has too much skill.
He's a master of the counterpunch. So, in effect, it turned out to be a trap for Hillary and McCain. If you look at his response, you see he's saying things he couldn't get away with before without looking like he's going negative. But because he is responding to an attack, he can hit them hard while looking like he's just defending himself.
A brilliant way of getting things like McCain's slowness on the mortgage crisis and Hillary's vote for the bankruptcy bill out on the table. My guess is that the voters of Pennsylvania are smart enough to see that Obama's criticisms of Clinton and McCain are more powerful than their criticisms of him.