My cousin told me that Rush Limbaugh said that this kid was in the Young Democrats and Rush is never wrong. Perhaps you should head back over to DU to spread filthy and Godless lies about Republicans.
According to this article from the St. Petersburg Times, he was a Young Republican:
Family paints pilot as being 'regular' teen
Sometimes Rush is wrong. That, or your cousin misquoted him.
Additionally, this letter contradicts the "he loved America" image that was promoted immediately after his suicide.
He had encountered Al-Quaeda members. The 9/11 attackers were instructed to "fit in" in America. Joining the Young Republicans could have been a part of this strategy.