Dave Weigel raises the question of why birtherism won't go away, but doesn't really answer it directly (in an otherwise worthy piece), so I'll offer an explanation. Here goes:
To really go mainstream, a conspiracy theory like this one needs a critical mass of people who believe it, to get the ball rolling. The birthers had that. They were fed by media figures (Limbaugh, Hannity, Beck) who would either endorse the conspiracy theory or tacitly encourage it, keeping the narrative alive. Then you had a political moment (campaign 2010), in which mainstream Republicans realized that their political survival depended on not antagonizing the extreme side of their party. They understood that those extremists were pretty much all on board with the conspiracy theory, so while few were willing to endorse it, most, including elite Republicans, were unwilling to denounce it (this is what our own Adam Serwer, in his excellent birtherism lexicon, calls "post-birtherism"). Now we move into a presidential primary campaign, where once again, the extreme side of the party is wielding outsize influence and making politicians tremble in fear.
So what you have is a situation in which there are almost no Republicans saying loudly that Obama is in fact not a foreigner, but there are two groups of people who are saying that: Democrats and journalists. Of course, conservatives have been told for years that both of these groups are made up of vicious America-hating liars, and you can't believe anything they say. So all the refutations in the world aren't going to convince them the conspiracy theory is wrong -- just the opposite, in fact.
Since 2008, the relevant political contests have been a midterm election, where motivating the base is essential, and a presidential primary, where persuading the base is essential. Since this really emerged as an issue, Republicans have been mostly looking over their right shoulder. That's where the danger is for them, so there has been little compelling reason to denounce birtherism and relegate it to the fringe. And their media figures, who wield far more influence than the left's media figures, live on the fringe anyway, so they're happy to keep pushing it.
It won't be until the general election of 2012 that a prominent Republican will have a powerful incentive to denounce birtherism. That guy, whoever he may be, will probably lose the election; his loss will be blamed within the Republican Party on his not being conservative enough; and throughout Obama's second term, birtherism will continue to gain adherents as Republicans continue to fear their base. It's not going anywhere.
