A July Pew Research survey found 12% of Americans continue to say Barack Obama is Muslim, virtually unchanged from 10% in March. This misperception is not limited to voters who oppose Obama. Identical percentages of Republicans and Democrats (12% each) think he is Muslim, and 37% of those who think he's Muslim intend to vote for him. A quarter of those polled don't know, but significantly those who have heard different things back McCain (48% to 33%) while those who just don't know back Obama (48% to 33%).
Source: Pew Research Center