[Update (6/30/10): Serious questions have been raised about the validity of Research 2000's polls. The results discussed below should thus be viewed as potentially suspect until the matter is resolved.]
Tom Jensen of Public Policy Polling continues his interesting polling on political misperceptions in a new national poll (PDF) that was conducted Sept. 18-21 and released today.
As in the New Jersey poll released last week, PPP's national poll includes contrasting "truther" and "birther" questions. However, Jensen consulted with me on the wording of the "truther" question, which I had criticized, and ended up selecting new wording I adapted from a Scripps Howard/Ohio University poll. The new question removes the ambiguity associated with the phrase "advance knowledge," which could be interpreted to refer to the August 2001 memo Bush received warning of the threat from Al Qaeda:
Old question: "Do you think George W. Bush had advance knowledge of the 9/11 attacks?"
New question: "Do you think President Bush intentionally allowed the 9/11 attacks to take place because he wanted the United States to go to war in the Middle East?"
Here are the poll results for the two misperceptions by party:
The primary finding is that the Obama birth certificate misperception has become far more prevalent among Republicans (42% no, 22% not sure) than the 9/11 misperception for Democrats (25% yes, 12% not sure). The percentage of Republicans who directly endorse the myth has increased substantially since the Daily Kos poll released in late July (which found 28% of Republicans said Obama wasn't born in this country and 30% weren't sure).
In terms of the 9/11 myth, the PPP results are generally consistent with what Scripps found in 2006, though reported levels of 9/11 misperceptions are lower among Democrats* (note: the Scripps results are juxtaposed below with the Daily Kos poll results referenced above):
The difference in 9/11 misperceptions between the two polls could be the result of variations in question wording (among other things, the original Scripps question refers to "people in the federal government" rather than President Bush) or differences in the response options (yes/no versus a scale of likelihood). 9/11 misperceptions may also have declined over time, particularly since Bush is no longer in office.