ATLANTA (Reuters) - The presence of an African American and a woman at the front of the race for the Democratic U.S. presidential nomination is shaking up the politics of identity in unpredictable ways, analysts said.

The confrontation between Sens. Hillary Clinton and Barack Obama boiled over at a debate on Monday when each attempted to undermine the credibility of the other's stand on social issues.
Unstated in that fight was a wider battle for the votes of demographic groups including white women, blacks and young people that could define the outcome of the Democratic contest in South Carolina on Saturday and influence other primaries.
"We don't have any good comparisons for how it (race and gender) is going to play out," said political science professor Robert Oldendick of the University of South Carolina on Tuesday. "We are in uncharted territory."
Candidates sometimes use what is known as the politics of identity to attract voters on the basis of factors such as race, gender, geography or age that stand outside of their policies.
Clinton would be the first female U.S. president, and Obama the first black, if elected in November to succeed President George W. Bush.
But it's one indication of the complexity of their situation that neither Obama nor Clinton has allowed that historic potential to form the basis of their identity as candidates, apparently wary of the pitfalls of such an appeal.
At the same time, both have challenged each other's natural constituency. Obama scored well among women in Iowa and around 40 percent of blacks in South Carolina are likely to vote for Clinton, according to polling data.
No black candidate has been elected to the Senate in the U.S. South for over a century, a fact seen as evidence that black candidates are at a disadvantage in a region where the legacy of slavery and racial segregation remains visible.
But the Democratic race has turned assumptions of the role of race and gender in presidential campaigns on their head and helped energize the voting base, commentators said.
"Hillary and Obama may be getting votes because of race and gender but as far as I can see they're not losing any," said former President Bill Clinton in a speech in Atlanta on Monday.
One effect is to allow them to "upstage" rival John Edwards, a former North Carolina senator, in terms of style and make him look old-fashioned, said Blease Graham, political science professor at the University of South Carolina.
Race works in Obama's favor in states such as South Carolina where around half the registered voters are black. Oldendick estimated it gave him a 15 point boost, while gender gave Clinton around a 4 percent boost among Democratic women.
But the role of identity would play differently in November because conservative politics makes much of the South infertile territory for Democrats, said Merle Black, professor of politics at Atlanta's Emory University.
Even white Democrats fare badly in most southern state races and the Democratic nominee in 2004, John Kerry, won just 30 percent of the white vote in the south, Black said.
The figure was a benchmark for judging the success of Obama or Clinton in November who would also be perceived negatively by many southern voters as big-government liberals, he said.
"In a general election campaign, if Obama's the nominee it wouldn't be his race that would drag him down in the south," said Black. "It would be his positions on the issues."