John Bazemore, Associated Press
Sen. Barack Obama, center, links arms with the Rev. Raphael Warnock, left, and associate pastor Shanan Jones during a song at an Atlanta church on Sunday.

CHARLESTON, S.C. — The three major Democratic candidates for president are expected to march in Columbia, S.C., today and verbally duel in Myrtle Beach tonight as the race for the White House focuses on a tight contest in South Carolina for the second week in a row — just a different political party this time.

Sens. Barack Obama and Hillary Clinton and former Sen. John Edwards, all vying for the crucial African-American vote in their party's primary, will join in a Martin Luther King Day march and rally this morning at the state Capitol in Columbia. Tonight they'll spar at the resort city in a nationally televised debate.

Palmetto State voters have barely paused from Saturday night's Republican race that ran so close television networks wouldn't call it until nearly all the votes were in. Now both parties are contemplating unsettled races that could stretch into spring or even a summer convention battle.

Clinton, who won New Hampshire, and Obama, who won Iowa, sprint into another close race after Clinton edged Obama in Nevada's caucus Saturday, raising the stakes for both of them this week.

"For five days, it'll be all out on both sides," said former Democratic National Committee Chairman Don Fowler, a Clinton supporter.

Former U.S. Sen. John Edwards of North Carolina returns to the state where he was born and which he won in 2004. This round, though, he arrives trailing in the polls, winless after three contests and after collecting only 4 percent in Nevada.

"I got my butt kicked. That is what happened in Nevada," Edwards told Wolf Blitzer on CNN's Late Edition Sunday.

Some Democrats are mixing the question of when he will drop out with speculation that he could play kingmaker with his delegates.

"The nomination won't be decided by win-loss records but by delegates," said a prepared statement by Edwards' campaign manager, former Michigan Congressman David Bonior, "and we're ready to fight for every delegate."

The three candidates are scheduled to join a 10 a.m. march starting at Zion Baptist Church in Columbia that will lead to a rally at the statehouse.

Columbia police say they are partnering with federal, state and local law enforcement to provide security at the march and rally, which is expected to attract thousands.

Later the candidates head to the coast for an 8 p.m. debate at Myrtle Beach's Palace Theater sponsored by CNN and the Congressional Black Caucus Institute. Past debates have provided some of the pivotal moments in the campaign.

Some Obama supporters, such as former S.C. Democratic Party Chairman Dick Harpootlian, suggest Obama could use the forum to fire back at Clinton after steady attacks from Clinton surrogates such as her husband, former President Bill Clinton.

"If there's going to be a stylistic change," Harpootlian said, "that's where Barack will make it."

Clinton emerged from Nevada a little stronger, just as U.S. Sen. John McCain's win among Republicans in South Carolina boosted his effort. In both cases, though, the second-place winner hung close enough to sustain the competition and push it down the line.

For Democrats, that's South Carolina. Even though the state has held a coveted early primary spot for a generation, the race usually has been winnowed down to two candidates by this time. Edwards' persistence and connections to South Carolina crowd the race.

Clinton and Obama will compete mightily for the support of African-Americans who make up half of Democratic primary voters. Race recently was at the center of some of the tensest days between the two candidates, a sparring that eventually calmed with a truce.