Brad DeLong approvingly quotes a reader making the following claim about misperceptions among Republicans:
12% of the country still thinks Obama is a Muslim. 8% thinks he faked his birth certificate. The new Washpost/ABC poll says that 22% of the electorate id's itself as GOP. Thus it is a fair inference that roughly half of declared Republicans are fringe lunatics--which explains why "respectable" conservative media outlets like National Review publish the Andy McCarthys and the Victor David Hansons, and why GOP politicians like Michelle Bachman and Steve King are now "mainstream" for the GOP.
DeLong's reader is way off. While the Muslim misperception is more widely held among Republicans, it's hardly an exclusively GOP phenomenon. The most recent Pew poll shows that 17% of Republican believe Obama is a Muslim -- not 50% -- along with 10% of independents and 7% of Democrats. No partisan cross-tabs are available for the dubious birth certificate poll, which was conducted for the fringe World Net Daily website, but I'd imagine they follow a similar pattern.
With that said, these levels of misperceptions are of course still much too high. For more on why these false beliefs tend to persist, see my new co-authored research with Jason Reifler and Duke undergraduates on correcting misperceptions about Obama's religion as well as my previous research with Reifler on the difficulty of correcting misperceptions (PDFs).