First, the claim is incredibly unusual compared to standard political irrationality. The last time this claim occurred was against Chester Arthur (US president 1881) - and John McCain (who was born in the Panama Canal Zone, outside the sovereign territory of the US) received no challenges to his eligibility for office.

Second, the ratio of intensity to plausibility is much higher that most American political irrationality - the long form has been released, after all. This suggests there's more than ordinary political mindkiller at play.

Why isn't "Politics is the mindkiller" sufficient for people to believe Obama is a space alien? Or for people to believe that John McCain isn't a natural born citizen, particularly given that he wasn't born in the US? "Politics is the mindkiller" isn't and shouldn't be able to account for just any negative belief that people hold about a candidate they don't believe in.

Why isn't "politics is the mindkiller" sufficient?

There's a danger in this being too broad an explanation, such that it doesn't actually explain anything. In this context, some ideas are so extreme, that simply using that explanation seems potentially insufficient. That said, while racism may be in play, there's some evidence otherwise that what is going on here is a combination of politics is the mindkiller with people who are already less connected to reality than others. Thus, for example, this probably explains why Alan Keyes was a birther (racism seems like it isn't likely to be relevant given that Keyes is black).

But at the same time, there's definite evidence for something other than just politics as the mindkiller. In particular, although some on the left did pick up on the birtherism early on (for example Phillip Berg), it didn't spread to the left-wing opponents of Obama in any substantial fashion, like it did to those on the right. In that context, the fact that Republicans in general are more racist seems robust. (For example, Republicans are much more likely than Democrats to have a negative opinion of interracial marriage or think it should be outlawed 1). And the possibility of a causal relationship has to be at minimum a located hypothesis. On the other hand, there are other possible hypotheses, such as the tendencies in the last few years for self-identified conservatives and Republicans to turn to their own news sources. While this occurs on both ends of the American political spectrum, it seems especially in the context of the last election and the response to people like Nate Silver and Sam Wang, that in the last two years it has occurred more on the right-wing. Moreover, it doesn't actually need to be more prominent on either side of the political spectrum to have had this sort of effect.

Overall, politics as mindkiller seems unsatisfactory in this context, but racism is definitely not the only possible other causal factor at work here. It seems likely that a variety of different factors are at play, and deciding how strong any given one of them is may be very tough.