Ad hominem abusive fallacies are fallacies of relevance.  The basic scheme for the fallacy type is:

P1: S holds that p

P2: S has some vice, X

C1:  Therefore, p is false (or unacceptable).

With my informal logic classes, I have the regular joke: Just because Brenda is a heavy drinker, that doesn't mean that she doesn't know much about politics — She may be a heavy drinker because she knows politics!  That gets lots of laughs, believe me.  But now, consider an argument of a different form, but composed of similar propositions:

P3: p is demonstrably false (i.e., there is sufficient and easily accessible evidence that p is false)

P4: S holds that p, despite P3

C2: Therefore, S has some vice X (where X = vices from simple stupidity to willful ignorance to suffers from ideological thinking)

Importantly, the argument has very similar claims as the ad hominem abusive, but it is of a different form — we are reasoning to S's vice, not from it.  Now, it is clear that this second kind of argument can be made hastily (as there is a big difference between being wrong and being stupid — that's the Fallacy of No Reasonable Alternatives, a species of false dilemma), but it does seem right that P3 and P4 are relevant to C2.  This second form of argument is one either (a) addressed to some third party about S or (b) addressed directly to S in order to request that S reform how she performs in argument regarding p (and perhaps other issues).

With the theoretical apparatus assembled, let's look at Steve Chapman's column, "Why Birtherism is Here to Stay," over at TownHall.com. 

There has never been a shred of persuasive evidence that Obama was born anywhere but Hawaii. But thanks to rampant paranoia and widespread credulity, the myth of his foreign origins gained currency among many people who should know better.

What is Chapman's explanation for this phenomenon — people who believe things that they should know better to not?

A poll taken after the release of his birth certificate showed 18 percent of those who have seen it still aren't convinced.  Something about this president impels many people to accept anything that is said about him, as long as it's unfavorable. . . .   Birthers don't dislike Obama because they think he was born abroad. They think he was born abroad because they dislike him. People of this bent don't proceed from facts to a conclusion. They prefer to reach a conclusion and then scrounge for any facts — or "facts" — that support it.  For them, being told Obama is a natural-born American is like being told he's a loving father and a loyal friend. They won't buy it because it doesn't confirm what they want to be true.

The logician and pragmatist C.S. Peirce called these sorts of patterns of thought 'pseudoreasoning,' and it looks very much like a form of rationalizing.  And the key to the effectiveness of these strategies of thought is that the people making errors with them are not exposed to the consequences of being wrong.  If you pseudoreason your way to believing that you can fly, you pay the consequences.  But if you pseudoreason your way to believing that the President of the United States is a Muslim Marxist AntiChrist, you make lots of friends (and if you stop believing them, you lose those friends).

This is surprising only if you think of political views as a matter of logical reasoning. For many people, they really aren't. They're a way of indulging emotional impulses without suffering painful consequences. . . . [I]f thinking Obama is a foreigner brings you closer to people you like, you come out ahead. Birthers would rather be wrong than be divided from their allies. So the fiction that Obama was born in Kenya will endure, and many Americans will hold fast to a ridiculous article of faith that has been conclusively refuted.

The thing is that this does amount to calling Birthers credulous, ideological, and cognitively blind.  Chapman forgot one thing more for his piece: directing readers to the comments for this piece!