NEW ORLEANS  The sharp surge in black turnout that Senator Barack Obama has helped to generate in recent primaries and Congressional races could signal a threat this fall to the longtime Republican dominance of the South, according to politicians and voting experts.

Should Mr. Obama become the Democratic nominee, he would still have to struggle for white swing voters in the South and in border states like West Virginia, where he lost decisively to Senator Hillary Rodham Clinton in Tuesday’s presidential primary. In West Virginia, where more than three-fourths of white voters chose Mrs. Clinton, 20 percent of the white voters said the race of the candidate mattered in their choice.

But in Southern states with large black populations, like Alabama, Mississippi and Virginia, an energized black electorate could create a countervailing force, particularly if conservative white voters choose not to flock to Senator John McCain, the presumptive Republican nominee. Merle Black, a political scientist at Emory University in Atlanta, predicts “the largest black turnout in the history of the United States” this fall if Mr. Obama is the nominee.

To hold these states, Republicans may have to work harder than ever. Already, turnout in Democratic primaries this year has substantially exceeded Republican turnout in states like Arkansas, Louisiana, South Carolina, Tennessee and Virginia.