There's any number of reasons to think that Bob Casey would be a bad VP choice for Barack Obama, but I want to make one very general point. It seems that a lot of people think the way to select a running mate is to figure out what the shortcomings of the presidential candidate are, and then pick someone who compensates. So if your nominee is Al Gore, tainted by his association to the sexually mischievous Bill Clinton, you pick moral scold Joe Lieberman. Or if you're running a stiff old Yankee like John Kerry, you pick dynamic Southern populist John Edwards. Or for Barack Obama, who has had difficulties with white working class voters in the primaries (why are we supposed to expect that this will carry over to the general?) you maybe pick Bob Casey who did well with the white working class in PA. This is not a good idea.
Voters weight the presidential candidate much more heavily than the vice-presidential candidate, as well they should. And when a presidential candidate chooses a VP to cover a weakness, it's considered an acknowledgement by the campaign that their presidential candidate has a weakness. Thereafter, the media is officially licensed to harp on that weakness. So now you reinforce the storyline: "Ordinary white folks don't and shouldn't like Obama!" or "John Kerry is a New England aristocrat with no ties to the common man!" Given the fact that the presidential candidate is more significant, a balancing strategy might actually end up moving perceptions of the ticket in the opposite direction.
So don't do it. Reinforce the good qualities of your candidate. For Obama, this might mean Kansas uniter Gov. Kathleen Sebelius, though I understand Stephen asking us not to steal his only hope for a Democratic Senator. Or maybe you want to tell a beautiful story of national healing and racial unity with the VP selection and pick a white Southerner who will constantly wax poetic about how great it is to be a black man's VP after growing up under segregation. (That was one of the things I found sweet about the Obama-Edwards possibility that probably won't be realized.)
