Politico traces it to Democrats, through the endlessly odd figure of Philip J. Berg, erstwhile 9-11 conspiracy theorist and putative Democrat:
The answer lies in Democratic, not Republican politics, and in the bitter, exhausting spring of 2008. At the time, the Democratic presidential primary was slipping away from Hillary Clinton and some of her most passionate supporters grasped for something, anything that would deal a final reversal to Barack Obama. [. . .]
But while the identity of the First Birther is lost to the mists of chain email, one of the first to put his name to the theory was Phil Berg, a former Pennsylvania deputy attorney general who had spent the previous years accusing President George W. Bush of complicity in the Sept. 11 attack.
In fact, this conclusion is not wrong, but the attempt at labeling is. Longtime readers will know Berg threw his lot in with the “PUMAs,” or diehard Hillary Clinton supporters, drawn more to her than to her agenda or beliefs. Recall the conspiracy theory’s original proponents, ObamaCrimes and TexasDarlin, etc.
It’s tough to conjure a label that’s at once broad and narrow enough to define that lot. While Democrats and believers in Democratic causes (gay rights, &c), many and even most PUMAs eventually became, by the time of the general election, a satellite group to Sarah Palin’s then-growing fan base. Since actions speak louder than words, it’s tough to call them Democrats; but if words matter at all, they’re hardly Republicans either. Better to call them simply “believers in the politics of emotion,” and move on.
Of course, this became, and remains, a Republican phenomenon.