Obama would have won South Carolina even if only 18% of the voters were black.
I used these pieces of data of estimate the candidates’ results had the African American turnout been much lower:
With 99% reporting:
Obama had 55% (295,091)
Clinton had 27% (141,128)
Edwards had 18% (93,552)
And Exit Poll Data:
55% of Democrat primary voters were African American
breakdown: Obama:78%, Clinton:19%, Edwards:2%
45% were white or other
breakdown: Obama:24%, Clinton:36%, Edwards:40%
This leads to an estimate of 529,771 total voters (for the top 3), of which 291,374 were cast by African Americans and 238,397 were cast by whites (and others).
If we assume the total number of white voters is constant and reduce the black turnout (but keeping the same candidate distribution), then we can estimate what the results would have been had the black voter turnout been much lower.
In fact, the turnout could have been as low as 18%, and Obama would have still won!
It would be sad to see the Clintons or media spin or suggest Obama’s good showing in South Carolina as solely due to the very strong African American turnout or the unique demographics in SC. Even in states with an average number of African Americans (and the same candidate breakdown*), Obama would still have won. Given that a vast majority of African Americans vote for democrats, and about half the US voters are democrats, and overall about 12% of the population is African American, then 18%-20% is a decent estimate for the total fraction of Democrats voting in a democrat primary.
*(Note: this assumption that there will be the same candidate breakdown in future states is of course not true, since Edwards’ support will not likely get any better than it was in his home state of South Carolina. The big question is: does Edwards take away votes from Clinton or Obama?)
Interestingly, Clinton was the most race-neutral candidate. She had steady support (20% of black voters and 36% of white voters), while Edwards had very strong white support (40% of white voters) and almost no black support (2% of black voters). Thus, if the black turnout was only 18%, then it would have been a virtual dead heat with all candidates getting about 33% (but Obama getting slightly more). Also, Clinton would not have won at any of the black-turnout levels.
In the 2004 Democrat Primaries in South Carolina, 47% of the voters were black, and in this primary, 55% of the voters were black. So, even with average black turnout in South Carolina, the vote would have still favored Obama by a very large margin.