I am thinking probably a socially conservative voter (based on the optics I am guessing probably either a middle class white Catholic or a black Republican (I hesitate to say "blue collar" given Casey's pro-labor chops)) who puts the sanctity of marriage in their top five issues. Rick Santorum made his name by being a strong defender of "traditional marriage" and Obama in 2008 took the PC position of the time that marriage was between a man and a woman but that he was in favor of civil unions. Fair enough, this was probably Casey Jr.'s position as well, but more often than not among swing voters perception is king. And I imagine quotes like this one made in October 2005 didn't help him among some of these types:
Which to a staunch anti-gay marriage advocate sounds more like "I am pro-gay equality, but I am too spineless to say I oppose it because I need religious voters!"
I mean it has to be that, because Santorum was likely on the right of every economic position Casey Jr. had and Casey Jr. was pro-gun enough to cancel out the "Muh NRA" voters.
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.
Of course, not that many people voted on the gay issue alone, which probably explains Santorum's borderline landslide loss.
