Why are so many Republicans delusional?

I was watching Washington Journal on C-SPAN yesterday (sorry, their video server is down, so no link), when one of the callers said he could never vote for Obama because "he's convinced that there's Muslim money behind him." This seems to refer to some vague notion that he's a Manchurian Muslim or in the pocket of a conspiracy run by some sinister, shadowy Jews Muslims. It's similar, of course, to the claim that was being pushed by FOX News and the Republican Party a few months ago that Obama was himself a Muslim. My mother works with a guy that's also convinced Arabs have been plotting to put Obama in office for years.

You'll find conservatives are pushing the line that Obama isn't even African-American, he's really Arab-American. I guess being black isn't Muslim-scary enough. Here we see it at GOPUSA:

[Obama] is the illegitimate son of a Kenyan Arab; not of an American Black (so we don’t owe him apologies for slavery); and stepson of an Indonesian Moslem. Obama was a Moslem studying the Koran in a Madrassa when he was 10 and listed as a Moslem in his Catholic school later on.

Here is The Conservative Voice:

Does Arab ancestry explain Obama's Palestinian sympathy and opposition to deposing Saddam Hussein by force? ...

The Senator's background is: Caucasian from his mother [and] Arab African from his father. Before all the Obamiacs jump on the answer, the Kenyan Obamas are listed in the Kenyan census as Arab African not as Tribal 'Black' African. His father's great great grandmother was a Tribal African.

"Therefore by ethnic lines the Senator is 50% Caucasian, 43.75% Arab, and 6.25% Black African (from where the Senator gets his skin pigmentation).

Here's a picture of Obama with his "non-black" father:

Here's one blogger of some renown again claiming Obama is of Arab descent. Here's a commentator at World Net Daily (color me surprised) insinuating, as recently as September, that Obama really is a Muslim.

These people are clearly delusional. This stuff is so easily debunked, and has been so thoroughly debunked that their claims fall apart under even the most trivial of examination. So why is it so persistent?

But my larger question is why is this sort of lunacy particularly a Republican phenomenon? You don't see influential forces on the Democratic side speculating about whether McCain was turned by the Viet Cong during his capture and that he really is a Manchurian candidate. You don't see people suggesting that Sarah Palin's having spent her life within sight of Russia has made her into a Communist, her socialist redistribution of Alaskan oil monies notwithstanding. Honestly, I don't have an answer, why is it that this kind of foolishness is a purely Republican phenomenon?