Another possibility are socially liberal Republican types who might view Casey as too pro-labor and too socially conservative to replace Santorum. Particularly on abortion they might see this as a net sum loss and that Santorum would at least be more "pro-growth" or "pro-business". Meanwhile in 2008 Obama was definitely seen as way more socially liberal but not liberal enough on economics to cause real damage. As well, McCain was widely seen as Bush's third term, which might turn off these voters.
This is actually how Santorum was re-elected in 2000. Dems nominated the SoCon Ron Klink, which sent the socially liberal suburban swing voters right into Santorum's arms. History will repeat itself if Dems nominate a joke like Chris Carney (luckily that's extremely unlikely.)
I think that because Democrats have become more uniformly socially liberal even a Bob Casey type now would have trouble getting the nod if he wasn't already a sitting senator.
