I think there's a somewhat decent argument to be made here. I consider Obama to be governing as a somewhat center-right politician, and although I know that's not universally accepted, I think if you look at his policies objectively, it's at least a fairly plausible statement. However, there are right wing factions that have, from the start, been fairly fanatically opposed to Obama (race might play some role, but given the political history of the last 10 years, I would not be surprised if the right would have reacted similarly to almost ANY democratic president). This fanatical opposition led to the rise of the Tea Party, which has moved the Republican party even further right and solidified the congressional republican strategy of opposition at any cost - clearly counter-productive to getting anything done.

Now, in my view, this radical far-right constituency would not have been nearly as vocal if a president such as McCain was in office. That's not to say that they wouldn't be there, but I don't think they would have nearly as prominent of a place in the Republican party, and I don't think that the Tea Party would exist at all. When a party is responsible for governing, it's hard for it to go too far off the deep end (I hope). Even far right Republicans would have had trouble vilifying a center-right Republican president, which is how I believe McCain would have governed. With a bit less opposition, such a president could and perhaps would have governed in a similar way as Obama has, but would have faced far less obstructionism. I'm not saying that McCain would have been a better president than Obama - I think the opposite is true, and Sarah Palin flat-out scares me. Instead, I just think that congressional Republicans have dug in their heels so far that even a Democrat who agrees with them on most fundamental issues is seen as a closet socialist Muslim out to destroy the US as we know it. With that kind of view and a minimum of 60 votes to pass anything in the Senate, it's hard to see how a moderate or even slightly right Democrat could govern successfully.

That being said, this year's crop of GOP candidates is pretty scary, and I don't know how well this logic applies. I think a figure like John Huntsman might be able to successfully tame the far right wing of his party while governing from a fairly centrist position, but he may be the only one. Romney is hard to get a read on - if he's just moved to the right rhetorically to get through the primaries, then he might govern in a reasonable manner. However, if he truly believes some of the things that he has said this summer/fall, or has boxed himself in too much with campaign promises, his presidency could be fairly catastrophic. In my view, the only "productive" reason to vote for the "greater" of two evils is if you think that the two candidates would in reality govern quite similarly but one would be unable to get anything done due to extreme obstructionism. If the two candidates would govern very differently, then it seems highly counter-productive to vote for what you see as the greater of two evils.