A bit of a long read, but David Sirota dissects a phenomena that people have been whispering about for months– the apparent race chasm that seems to be the surest way to predict Democratic primary results:

“When the black population is really small, racial polarization is small enough that Obama can win, and when the black population is large, any polarization is drowned out by the overwhelming size of the Democratic black vote,” says Schaller, who recently authored the book Whistling Past Dixie analyzing demographic voting trends. “But in the middle range, polarization is sizeable enough that black voters cannot overcome it, and these are the states where she wins.”

Here’s the graph Sirota provides that pretty convincingly illustrates the point:

On the left of the graph, among the states with the smallest black population, Obama has destroyed Clinton. With the candidates differing little on issues, this trend is likely due, in part, to the fact that black-white racial politics are all but non-existent in nearly totally white states. Thus, Clinton has fewer built-in advantages.

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On the right of the graph among the states with the largest black populations, Obama has also crushed Clinton. Unlike the super-white states, these states—many in the Deep South—have a long and sordid history of day-to-day, black-white racial politics, with Richard Nixon famously pioneering Republican’s “southern strategy” to maximize the racist segregationist vote in general elections. “But in the Democratic primary the black vote is so huge [in these states], it can overwhelm the white vote,” says Thomas Schaller, a political science professor at the University of Maryland—Baltimore. That black vote has gone primarily to Obama, helping him win these states by big margins.

The good news for Obama is that there’s little evidence the race chasm phenomena would carry over into a general election population, and even if it did it would mean that traditionally red Midwest and Mountain states like Iowa and Colorado could be up for grabs.

The bad news is that Pennsylvania’s demographics fall smack into the middle of the chasm– a point I made, as a former resident of the state, a few weeks ago. And two more traditional general election swing states, Ohio and Florida, also fall within the chasm– a fact that would be offset by gains in red states outside the chasm. But again, that’s only if the phenomena has any effect beyond Democratic primary voters.

The one general election certainty of a Barack Obama candidacy is that the electoral map will be completely rewritten. As tempted as the networks will be in November to focus on the results in Ohio, Florida, and Pennsylvania, the real battleground– as illustrated in this very early electoral analysis conducted by SurveyUSA– will likely end up being states like Texas, Virginia, Nebraska, and South Carolina.