They're people who believe something crazy, and may or may not be crazy in other parts of their life. Some people claim (with a straight face) that Obama's mother had a secret one-night stand with a national civil rights leader who lived several thousand miles away. That's pretty crazy.
As far as intelligence goes, it's not like you find many astrophysicists who say the moon landing was faked. Then again, Birthers can't seem to find any Constitutional professors who are willing to vouch for their claims about the President needing two citizen parents. But I'm sure that's because of some kind of conspiracy of silence, because the Birthers' conspiracy is real, you know.
Dr. David Ramsey.
Apart from that, you are arguing the "Fallacy of Authority." Look it up if you do not know what that is. (An Argument isn't valid because of WHO agrees with it. It must be accepted or rejected on it's own merits. The Church didn't agree with Galileo, yet it was Galileo who was correct.)