Interesting... might be something to this...

TPM post Damn Statistics gets into the race issue for Obama...

What I take to be implicit in Sirota's argument is that racially polarized voting increases with the size of the black population in a given state. That leaves Obama winning a lot of states with few blacks. But once the black population gets into the high single digits, racialized voting kicks in and Obama then can't get enough of the white population to win. Only when blacks approach 20% of the population does the black population get large enough to make up for and often overcome the increased white resistance to voting for Obama. (After all, I don't think Sirota is saying that Mississippi and Louisiana are models of racial harmony.) Of course, these are Democratic primaries, not general elections. And that is the key. Because that means that in most cases the percentage of the black electorate is roughly double what it is in the general election.