Salon's Steve Kornacki writes on racial and political roots of birtherism:
[A]s Obama seeks to put all of the zany conspiracy theories to rest for good, it's worth remembering that there's a broader phenomenon that birtherism grew out of: The right's instinctive, aggressive rejection of Democratic presidents. [...]
The story of **Bill Clinton**'s presidency is presidency is a perfect example. The attacks from the right -- which began even before he took office -- weren't about his birthplace or citizenship status, but they were designed to fill the GOP base with rage and resentment that a usurper was somehow in the White House. In the early days of Clinton's presidency, the Internet as we now know it didn't exist and cable news consisted only of CNN (which hadn't yet been spooked by the rise of Fox). But through talk radio, newsletters, and widely-circulated videotapes, the right devised and promoted one preposterous, inflammatory claim after another: that the Clintons had arranged for Vince Foster to be killed, for instance, or that they'd been involved in drug-running in Arkansas.
You should read the whole post, because it really illustrates the extent to which the Right has an enduring problem with the basic legitimacy of Democratic presidents. A President Hillary Clinton or a President Edwards would have faced a similar torrent of conspiracies and accusations. The difference, for President Obama, is that these conspiracies have less to do with something he's done, and more to do with his basic identity. For birthers, Obama is illegitimate by virtue of who he is, much less his liberalism.
