And yet since the South began to shift away from the Democrats in the 1960s, it has become one of the biggest and reddest of the Republican strongholds. In the last two presidential elections, the Democrats failed to carry any of the Southern states. Although recent Democratic nominees have typically gotten about 9 out of 10 of the votes of Southern blacks, they still need a substantial chunk of the white vote to prevail. Political scientists put that figure at close to 40 percent, though it depends on the state, and the Democrats have rarely gotten it.

Even after selecting a Southerner, John Edwards of North Carolina, as his running mate in 2004, Senator John Kerry of Massachusetts drew 29 percent of the white vote in the region (17 percent in the Deep South). In 2000, Al Gore got 31 percent, even losing his home state, Tennessee.

The only times since 1972 that the Democrats have carried more than a third of the Southern white vote, according to exit polls, were when Jimmy Carter or Bill Clinton, both Southerners, were atop the ticket. In 1996, for example, Mr. Clinton got the votes of 36 percent of Southern whites and 87 percent of Southern blacks, and carried 5 of the 13 Southern states.

Mr. Obama’s Southern strategy relies on significantly increasing black registration and turnout, as he did in the primary season. Mr. Hildebrand said that by some estimates there are 600,000 unregistered black voters in Georgia alone. The higher the black share of the vote, the lower the requirement for garnering white votes. But the Obama camp argues that it can increase its share of the white vote as well by focusing on younger, more progressive whites.

Democratic candidates have typically written off many Southern states early in the process. But when Democrats give up the South, they need to win 70 percent of the rest of the electoral votes, said Merle Black, an expert on Southern politics at Emory University. And they often subject candidates running for lower offices in the region to fierce political headwinds: it is hard for a statewide candidate to prevail when his party’s presidential nominee loses by double digits.