There was nothing like this for ANY President in living memory... all of which just happen to be white.
Depending on how grounded in reality a Birther still is, they expect Obama to be replaced either by Biden or by McCain... both of which are white.
It´s a process of elimination. There is no evidence that Obama is ineligible... so evidence cannot be the driving force. There is no reasonable (or unreasonable) doubt about the evidence that he is eligible... so reasonable (or unreasonable) doubt cannot be the driving force. Having him declared ineligible would swap a black President with a white President... therefore racism is a possible driving force.
Note that I wrote "there is no doubt"; what there is, coming from the Birthers, is blind, mindless denial. It´s important to keep the difference in mind.