++Addition++ Per Agnostic's cosmetic suggestions, the graph presented in a way more suggestive that the black voting percentage contributing to the increased Republican support among whites.
In discussing some examples of where individual and state-level electoral trends differ, I've made note of the fact that blacker states are more likely to vote Republican than less black states are. Given the overwhelming propensity of blacks to vote Democratic, this obviously suggests the black population's effect on the white vote in a state is stronger than it is on the entire state's electoral leaning (obvious though it is, I failed to present it in this way--Orwell's remark "To see what's in front of one's nose requires a constant struggle," appropriately describes me, except that my ignorance is never willful).
The direction of a state's white vote and the percentage of the electorate that is black correlate at a firm .55 (p=0)*. For each percentage point increase in black representation among a state's electorate, McCain improved his support among whites by three points.
Yet looking at all states dilutes the potency of the black population's effect on the white vote. Whitebread red (ie North Dakota) and blue (ie Vermont) states, where racial concerns are mostly abstractions, attenuate the relationship. If only the top 25 states by percentage of the population that is black are included, the correlation jumps to .72.
I speculate that in blacker states like Mississippi, whites are less likely to vote Democratic in part because doing so means supporting wealth transfer policies that primarily benefit blacks at the expense of whites. It's a less abstract moral exercise and more of a real world sacrifice than it is in say, brainy Vermont, where leftist quasi-socialism doesn't have anywhere near the same pathological consequences as it would down South. Also, in lily-white states, most welfare beneficiaries are whites who are presumed to be only momentarily down on their luck.
It's easy to conceive how this pattern could be accentuated further in the coming four years, depending on economic conditions and the agenda President Obama pursues.
* This does not include DC, an entirely urban outlier without the same state-level concerns voters in the rest of the country have. It bucks the trend, with whites voting 7 to 1 in favor of Obama.