The bottom line is that Edwards draws white voters from Clinton while taking virtually no black voters from Obama.
Obama is dominating Clinton among African-Americans nationwide, and even stronger in states where campaigning has actually taken place. Edwards draws very little of the African-American vote from Obama, but is competitive for white southern votes. This means that in states like South Carolina (Jan 26th) and Georgia (Feb 5th), Obama's lead is largely dependent on Edwards staying in the campaign. In Alabama (Feb 5th), and Missouri (Feb 5th), Obama has a chance to win both states, but probably only if Edwards stays in the campaign and stays in double-digits. Obama winning Kansas (Feb 5th) is probably also dependent on Edwards staying in the campaign. Further, Obama's delegate totals in all of these states will be improved by Edwards staying in the campaign. Further, while his winning or losing in Arkansas, Connecticut, Illinois, New Jersey, New York, and Tennessee is not dependent on Edwards staying in the campaign, Obama's delegate totals from these states will probably be better with Edwards in the campaign than with Edwards out of the campaign. In every case, Edwards will take a larger bite out of Clinton's advantage among white voters than he will from Obama's advantage among African-American voters.