The problem with doing so now is that the DNC can send in Hillary, who gave some votes to McCain once he picked Sarah. If Hillary runs now, what will happen to those votes? Will McCain lose them back to Hillary? Will she keep all of Obama’s votes or will some go to McCain and make up the difference?
If Obama is disqualified now, a large number of his supporters will stay home, and be unmotivated to get out the vote for Hillary. It might even be the case that unrest would make opening the polls in the inner city problematic on Nov 4.