Basically, birtherism (i.e. the idea that Barack Obama was born in a foreign country and ergo is unqualified to be President of the United States) has become a part of Republican orthodoxy, similar to what opposition to marriage equality is becoming. It is just one test in a series of signifiers of partisan loyalty and ideological purity. In other words, if you are a Republican you must believe that Obama is not a Christian, was born in a foreign country, that taxes must never ever go up, global climate change is a hoax (or not caused by humans) and that marriage is (and has always been) an entity created by God that consists of one man and woman.
The problem with this orthodoxy is that most of these statements are beliefs, and they are not supported by actual facts. Thus the nature of political debate changes to one between one side who provides its own facts and the other side which just looks on in disbelief, unsure how to even begin such a "debate." There can be no debate between orthodoxy and reality.
This really does not bode well for future actions that our political system must take in order to solve our nation's pressing problems.
The poll was conducted April 26 to May 2, 2012 and has a margin of error 3.3 percentage points.
Hat/tip to Kevin Drum.