There's been a lot of talk since the Palin pick was announced of how this has rapidly animated the Republican Christianist base.
But if my own response is anything like those of others who gave to Obama during the primary season--but have held off donating since--then maybe the Republican base isn't the only one getting motivated.
McCain could have gone any number of directions with his campaign and his VP selection, but that he's chosen the path he has, has made the prospect of his and his party's continuing in the executive branch of government catastrophic.
Sure, maybe that would have happened anyway, that no matter how "centrist" McCain's public persona, his actual record is anything but, and activist Democrats would have gone all out for Obama eventually. But McCain's choices, both in campaign style and in executive decision-making, are reinforcing the travesties of government mangling and abuse of power during the past eight years. At least in its original roll-out period, George W's promise of a "compassionate conservatism", combined with the experience of Dick Cheney, suggested something resembling competency and deliberateness about the 2000 ticket (even if it didn't turn out that way). The 2008 team, however, is arguably on its face, the most ideological and reckless pairing since at least 1968. And where past candidates, like to a certain extent Obama, have campaigned to their party's base in the primaries and returned to the center during the general, McCain appears to be doing the opposite.
I suspect many of us who have been nearly lulled to sleep by Obama's turn to the center, have been sufficiently stirred up now.
Update: Here's some empirical evidence in support of the speculation. Maybe there's something to be said for having your opponent go last in the convention cycle. (h/t Andrew Sullivan).