Many libertarians voted for Obama – or had a pair-wise preference for him – in order to prevent (or express disdain for) a McCain presidency and all that might mean (for example, the neoconservative agenda – especially in foreign policy). They also hoped that some time in the wilderness would be good for the Republican party and serve as appropriate punishment for its big government conservatism turn (This hope has been somewhat borne-out). In terms of the latter, Alex Tabarrok argued:
Lack of power is no guarantee of virtue but Republicans are a far better – more libertarian – party out-of-power than they are in power. When in the wilderness, Republicans turn naturally to a critique of power and they ratchet up libertarian rhetoric about free trade, free enterprise, abuse of government power and even the defense of civil liberties. We can hope that new leaders will arise in this libertarian milieu.
But given what has occurred over the last year and a half, is McCain starting to look a lot better every day? Or is the country still going to be better off in the long-term despite electing Obama (given we avoided McCain and Obama’s leftist governance could lead to a resurgence of a more libertarian Republican Party)?
Of course, individual voting is strategically irrational, so don’t feel guilty if you regret voting for Obama.
But if you could go back in time and be the marginal voter between Obama and McCain (as if this could ever happen in a large-scale election, let alone one in a system in which anything near a close decision would be litigated), what would you do?
PILEUS 
This doesn’t have to be a time-traveling thought experiment — just look at the primary ads McCain is running now (e.g., “complete the dang fence”).
I wouldn’t have time to vote. I would be too busy short selling stocks.
If I could have been the deciding vote in the Republican Primaries, however, I would have chosen Ron Paul. Not that he would stand a chance in the general election but it would have sent a message.
I would then do what I could to save Sarah Connor
I suspect McCain’s policies would have been harder to roll back in the long-term. He’d probably be conscripting young adults by now – he doesn’t have a small government bone in his body and shows brash disregard for natural rights. And given the political climate, he would have received less opposition than Obama is seeing, given the same actions.
I wrote in Ron Paul in the general election, and would do it again. If I really had magical time-travel powers I’d go back and teach 18th Century statesmen about Condorcet voting methods.