Some 18% said the president was a Muslim, up from 11% in March 2009, according to the Pew Research survey of 3,003 Americans.
Among Republicans, that number was 34%. Just a third of those quizzed correctly identified Mr Obama as Christian.
Polling was done before 13 August when Mr Obama defended Muslims' right to build an Islamic centre by Ground Zero.
In ten years at this pace, the US won't be able to feed and clothe itself. And the public discourse will look like something between the rhetoric of the Civil War and the Inquisition, but on bathtub gin and automatic weapons.