Senators Obama and Clinton collided today in a racially charged South Carolina primary, prelude to the February 5 coast-to-coast competition for more than 1,600 national convention delegates.

A former senator, John Edwards, was the third contender on the ballot, hoping to benefit from the acrimony between the other two.

South Carolina offered 45 Democratic National Convention delegates, as well as the campaign’s first indication of Mr. Obama’s political appeal in a state with a large black population.

Mrs. Clinton hopes to become the first woman to occupy the White House, and Mr. Obama is the strongest black contender in history.

After playing a muted role in the earlier contests, the issue of race dominated an incendiary week that included a shift in strategy for Mr. Obama, a remarkably bitter debate, and fresh scrutiny of President Clinton’s role in his wife’s campaign.

Polls opened at 7 a.m. and election officials reported no problems with voting machines as there were in last week’s Republican primary after one county’s electronic machines failed to function properly.

“Everything’s going smoothly,” a state Election Commission spokesman, Chris Whitmire, said.

Mr. Obama spent this morning greeting South Carolinans at a predominantly black Baptist church and at historically black Benedict College in Columbia, S.C. Later, he surprised lunchgoers at Harper’s restaurant who posed for pictures with him and shook his hand. A restaurant patron, Scott Boyd, told Mr. Obama he’s been a lifelong Republican but did not vote in last week’s Republican primary so that he could vote for the Illinois senator in the Democratic primary.

“There is a charisma about him. He seems to represent change. He’s an aggregator, he pulls people together,” Mr. Boyd said in an interview.

Mrs. Clinton and daughter Chelsea greeted voters at a Shoney’s restaurant in Columbia. Mrs. Clinton hoisted babies and wedged into booths to chat with patrons surprised to have their breakfast interrupted by a presidential candidate and a huge media horde.

A patron, M.J. Hassell, 66, said she had agonized over whom to support but finally sided with Mrs. Clinton.

“I did like Obama’s stand on being against the war from the start. But Hillary’s got the experience we need,” Ms. Hassell said.

Mrs. Clinton and Chelsea later sat down for breakfast with Rep. Laura Richardson, a black congresswoman of California, who was campaigning in South Carolina for the former first lady.

In a separate morning stop at a diner, also in Columbia, Mr. Clinton had grits, coffee, three helpings of eggs, and a swipe at the Mr. Obama campaign’s mantra of change, which is juxtaposed with his wife’s emphasis on experience.

“Since 100% of medical malpractice is committed by doctors, next time you need surgery, go to an electrician,” the former president quipped in the restaurant that had about three dozen people.

He later went to a polling place where he signed autographs and posed for pictures.

Mr. Edwards also hit a cafe and polling place. He acknowledged his prospects for winning the primary are dim, but he told reporters at a Mount Pleasant, S.C., restaurant that he’s still in the race, no matter how he fares today.

“The role I want to play is president of the United States,” Mr. Edwards said.

Mrs. Clinton and Mr. Obama swapped accusatory radio commercials earlier in the week.

The former first lady aired an ad saying Mr. Obama had once approved of Republican ideas. His camp responded quickly: “Hillary Clinton will say anything to get elected.” First she, then he, pulled the commercials after a couple of days.

Given the bickering, Mr. Edwards looked for an opening to reinvigorate a candidacy all but eclipsed by the historic campaign between Mr. Obama and Mrs. Clinton. He went on the “Late Show with David Letterman” at midweek to say he wanted to represent the “grown-up wing of the Democratic party.”

That was one night after a finger-wagging debate in which Mr. Obama told Mrs. Clinton he was helping unemployed workers on the streets of Chicago when “you were a corporate lawyer sitting on the board at Wal-Mart.”

Moments later, the former first lady said she was fighting against misguided Republican policies “when you were practicing law and representing your contributor … in his slum landlord business in inner city Chicago.”

Each side accused the other of playing the race card, sparking a controversy that frequently involved the former president.

“They are getting votes, to be sure, because of their race or gender. That’s why people tell me Hillary doesn’t have a chance of winning here,” Mr. Clinton said at one stop, strongly suggesting that blacks would not support a white alternative to Mr. Obama.

Clinton campaign strategists denied any intentional effort to stir the racial debate. But they said they believe the fallout has had the effect of branding Mr. Obama as “the black candidate.”

By week’s end, one poll indicated that Mr. Obama’s support among whites in the state had dropped sharply, a danger sign for him in the rush of primaries and caucuses that begins on February 5.

The former president’s efforts drew criticism from Mr. Obama’s allies as well as Democrats neutral in the race. “Not presidential,” an Obama supporter who was Senate majority leader at the time of Mr. Clinton’s impeachment nearly a decade ago, Tom Daschle, said.