From the high plains of West Texas to the Atlantic Coast of Georgia, white voters opposed Mr. Obama’s re-election in overwhelming numbers. In many counties 90 percent of white voters chose Mitt Romney, nearly the reversal of the margin by which black voters supported Mr. Obama.

While white Southerners have been voting Republican for decades, the hugeness of the gap was new. Mr. Obama often lost more than 40 percent of Al Gore’s support among white voters south of the historically significant line of the Missouri Compromise. Two centuries later, Southern politics are deeply polarized along racial lines. It is no exaggeration to suggest that in these states the Democrats have become the party of African Americans and that the Republicans are the party of whites…

It is impossible to discuss Mr. Obama’s weakness among Southern whites without mention of race. It is surely a factor, and perhaps even a large one. Mr. Obama performed significantly worse than John Kerry among Southern whites, even though both were Northern liberals and 2008 was a far better year for Democrats than 2004. (The estimates are derived from census and exit poll data). And the pattern of white support in the 2012 presidential election is an eerie reversal of post-Reconstruction presidential elections, when Jim Crow laws rendered blacks ineligible to vote and Democrats won the so-called Solid South by similar margins.

But it is hard to know the extent to which racism is responsible for Mr. Obama’s weakness. After all, Mr. Obama is not the only Democrat to perform so poorly in recent years.