New Orleans —- The sharp surge in black turnout that Sen. 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 Obama become the Democratic nominee, he would still have to struggle for white swing voters in the South and in border states such as West Virginia, where he lost decisively to Sen. Hillary Clinton in Tuesday's presidential primary. In West Virginia, where more than three-fourths of white voters chose Clinton, 20 percent of the white voters said the race of the candidate mattered.

But in Southern states with large black populations —- Georgia, Alabama, Mississippi and Virginia —- an energized black electorate could create a countervailing force, particularly if conservative white voters fail to flock to Sen. John McCain, the presumptive Republican nominee.

Merle Black, a political scientist at Emory University, predicts "the largest black turnout in the history of the United States" this fall if Obama is the nominee.

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

Some analysts suggest North Carolina and Virginia may even be within reach for the Democratic nominee, and they point to the surprising result in a congressional special election in Mississippi last week.

With the strong support of black voters, a conservative white Democrat, Travis Childers, scored an upset in that race, in a district held by Republicans since 1995. Kelvin Buck, a black state representative who helped the Childers campaign, said he saw a "level of enthusiasm and energy" that he had not seen before from black voters —- significantly motivated by a recent Republican anti-Obama campaign.

Black voters turn out

The numbers appear to bear that out. In one black precinct in Amory, Miss., the number of voters nearly doubled, to 413, from the 2006 congressional election, and this for a special election with nothing else on the ballot. Meanwhile, in a nearby white precinct, the number of voters dropped by nearly half.

A similar increase in black voters has been evident in Southern states with presidential primaries. In South Carolina, the black vote in the primary more than doubled from 2004, to 295,000, according to exit polls. In Georgia, it rose to 536,000 from 289,000.

One expert on black politics, David Bositis of the Joint Center for Political and Economic Studies, called those numbers "almost astounding." Black turnout shot up in Maryland, Virginia and Louisiana, even after Hurricane Katrina had driven many Louisianans out of state.

Larry Sabato, director of the Center for Politics at the University of Virginia, said, "This is going to encourage the purple-ization of red states."

Black voters made up a larger percentage of Democratic primary voters this year in several states than in the last two presidential election years, according to exit polls by Edison/Mitofsky for the National Election Pool of TV networks and The Associated Press this year and in 2004, and by the Voter News Service in 2000. In Maryland, black voters rose to 47 percent of the total, up from 35 percent in 2004 and 28 percent in 2000.

Ronald Walters, a professor of government and politics at the University of Maryland who worked for the 1984 presidential campaign of the Rev. Jesse Jackson, said of Obama, "He's generated a tremendous force ... outside the electoral system."

Risky tactics?

Still, it would take a shift in the electoral dynamic —- a substantial stumble by McCain, for instance —- for Obama to put in play a state like Mississippi, where whites gave John Kerry only about 15 percent of their vote in 2004. Even with a heavy black turnout, Bositis estimated that Obama would have to increase his white percentage by at least a third, to about 20 percent, to win the state.

"I don't anticipate him winning Mississippi," Bositis said.

Many of the votes for Childers —- an anti-abortion, pro-gun-rights Democrat —- were from whites who will in all likelihood pull the lever for McCain in November, analysts say.

Bruce Oppenheimer, a political scientist at Vanderbilt University, said the question is not so much whether Obama would carry Mississippi, but whether he would force Republicans to spend time and money in the state.

Yet one sure lesson of the northern Mississippi congressional result is the use of Obama as an electoral tactic. At best it is a double-edged sword. At worst, it is a guillotine for Republican candidates in areas with substantial black populations, such as the Mississippi district won by Childers, where 26 percent of voters are black.

 The New York Times 
HOUSE SEATS GAINED BY DEMOCRATS 
Tuesday's Democratic victory in a special election for a Congressional seat in Mississippi was the party's third consecutive win in a solidly Republican district this year. 
Percentage of votes received by the Republican candidate 
Illinois, 14th District
Special election to fill House seat, 2008: 47%
House race, 2006: 60%
2004: 69%
2002: 74%
Presidential race, 2004: 56%
2008 Democratic primary winner: Obama 63% of votes 

Louisiana, 6th District
Special election to fill House seat, 2008: 46%
House race, 2006: 83%
2004: 72%
2002: 84%
Presidential race, 2004: 59%
2008 Democratic primary winner: Obama 60% of votes 

Mississippi, 1st District
Special election to fill House seat, 2008: 46%
House race, 2006: 66%
2004: 79%
2002: 71%
Presidential race, 2004: 62%
2008 Democratic primary winner: Clinton 50% of votes 

Sources: Congressional Quarterly; each state's secretary of state