Despite the South’s continued economic and population growth, there’s not much sign that the gap between the South and the rest of the country is poised to narrow. Young voters have moved the rest of the country abruptly to the left on issues like gay marriage and immigration, but young Southern whites are just as conservative as their parents and grandparents. If they remain so, the gap between the South and the rest of the country could grow further. And although an influx of Northerners has transformed metropolitan enclaves of Virginia, North Carolina and Florida into liberal bastions of Yankee expats, much of the South remains largely untouched.

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. Some white Democratic candidates, like Virginia Gov. Terry McAuliffe, have done worse than Mr. Obama among Southern whites. And Mr. Obama’s losses are part of a longer-term trend. Mr. Kerry, for instance, performed worse than Al Gore, who even fared worse than Michael Dukakis among Southern whites.

The collapse of Democratic support among Southern whites threatens the party’s ability to control government and enact its agenda. Democrats will find it extremely hard to retake the House without reclaiming the majority white, Southern districts once held by the now vanquished group of Democrats known as the Blue Dogs. This November, Southern whites could easily deny Democrats control of the Senate by dismissing Democratic incumbents in North Carolina, Arkansas and Louisiana.

Over all, though, the pattern is also worrisome for Republicans. The party’s big gains among white Southerners do little good in the Electoral College, which rewards a geographically broad electoral coalition. A stronger Republican showing in Oklahoma or South Carolina makes red states only more red; it doesn’t give the Republicans additional electoral votes.

The problem for Republicans is that the Democratic weakness appears confined to the white South. Even though some analysts suggested that Mr. Obama was historically weak among white voters more generally, he fared better than recent Democratic nominees among white voters outside of the South. That’s how he won battleground states like Iowa, Colorado, Wisconsin and New Hampshire. Whatever is causing Republicans to excel in the South, whether religion or race, just isn’t helping them elsewhere.

Moreover, the Republican Party’s increasingly Southern character makes broadening its appeal more challenging. A record 41 percent of Republican voters in the 2012 election hailed from the South. Those voters elected more than half of all House Republicans in 2012 — the first time that Southerners have represented a majority of the House Republican Caucus. They have since blocked establishment-led efforts on an immigration overhaul and voted to shut down the government by an 88 to 25 margin in October, after an effort to repeal the Affordable Care Act predictably failed.