(CNN) -- After a week of at times bitter campaigning, Sen. Barack Obama faces a crucial test of his support from within the party Saturday as South Carolina Democrats head to the polls in a race that features black voters for the first time this presidential primary season.
Sen. Barack Obama campaigns Thursday in North Charleston, South Carolina.
The South Carolina primary is the first contest in the South for the Democrats, and more than half the primary voters are expected to be African-American.
"South Carolina is important for Democrats for the same reason it's important for Republicans: It's the state where the base speaks," said CNN senior political analyst Bill Schneider. "In the Republican case, that means conservatives. For the Democrats, that means African-Americans."
A victory in South Carolina is particularly critical to Obama of Illinois, who won in Iowa but lost to Sen. Hillary Clinton of New York in New Hampshire and Nevada. Polls have Obama leading Clinton in South Carolina, with former Sen. John Edwards of North Carolina in third place.
"Obama's support among African-American voters gives him more legitimacy," Schneider said. "Obama has been doing well with young voters, independents and educated upper-middle-class liberals -- the NPR vote. Winning the black vote by a solid margin means Obama has standing with the Democratic Party's base."
"The closer Hillary Clinton comes to splitting the black vote with Obama, the easier it will be for her to say that she and Obama share the support of that base," Schneider added.
The South Carolina primary is also the last time Democratic voters will weigh in with any significance before "Super Tuesday" February 5, the day nearly two dozen states will hold either primaries or caucuses -- including such delegate-rich states as California, New York, New Jersey and Illinois.
A McClatchy-MSNBC poll conducted January 22-23 reported Obama leading with the support of 38 percent of the likely Democratic primary voters polled. Clinton was in second place with 30 percent, and Edwards was backed by 19 percent.
Among black primary voters, Obama had a more significant lead over Clinton, 59 percent to 25 percent, but Obama is only backed by 10 percent of white voters, the poll found. Among whites, Edwards and Clinton are in a statistical tie, with Edwards backed by 40 percent and Clinton suported by 36 percent.
The lack of support from white voters could be a concern for Obama in the future.
"The concern all along has been the possibility of Obama, in spite of his broad, non-racial appeal, running poorly among whites," said Thom Mann, a political analyst at the Brookings Institution.
The McClatchy-MSNBC poll's margin of error was plus-or-minus 5 percentage points.
An American Research Group poll of likely primary voters conducted over the same period showed a similar lead for Obama. In that poll, Obama led Clinton 45 percent to 36 percent, with Edwards coming in third with 12 percent. The poll's margin of error was plus-or-minus 4 percentage points.
Polling is not an exact science, however. Before the New Hampshire primary, many polls showed Obama beating Clinton by approximately 9 percentage points. But Clinton won, beating Obama 39 percent to 37 percent.
That the South Carolina primary has mostly come down to a race between Obama, a black man, and Clinton, a white woman, has put the issues of race and gender front-and-center.
Obama's wife, Michelle, was brought into the race conversation Wednesday when she was asked what role it played in the election. Watch Michelle Obama say her husband is the right choice »
"My deep hope is that people will base their decision on who they think they can trust, who's got a vision for the country, who's bringing a different tone to politics, who's going to really take this country in a different direction," she told a News 14 Carolina reporter. "And quite frankly, I think the only person who comes close to that is Barack, and he happens to be a black man."
Former President Clinton, who has been tirelessly campaigning for his wife in South Carolina, said race and gender considerations hadn't cost his wife or Obama any votes so far.
"I love this primary because it looks like we are going to nominate an African-American man or a woman and they aren't going to lose any votes because of their race or gender," Clinton said Friday in Spartanburg, South Carolina. "They're picking up some because of it, but that's to be expected."
During the week leading up to the primary, South Carolina voters have witnessed some of the most bitter exchanges between the Obama and Clinton camps this primary season, with each camp accusing the other of dirty tricks and spreading lies.
The former president and the Obama campaign have traded barbs almost daily. In a debate Monday, Obama himself seemed frustrated by the attacks by Bill Clinton, saying, "I can't tell who I am running against sometimes."
In a radio ad aired in South Carolina but later pulled, Bill Clinton questioned comments Obama made to a Reno newspaper's editorial board in which he called former President Reagan a "transformational" figure, unlike Bill Clinton, and asked if Obama's praise for the former Republican president meant he supported many of Reagan's policies. Watch the air war in South Carolina »
The Obama camp accused Bill Clinton of twisting his words and ran its own radio ad in which the announcer says "Hillary Clinton, she'll say anything and change nothing."
Bill Clinton's high-profile appearances in South Carolina and his attacks on Obama have raised concerns within the black community and questions about the 42nd president's role on the campaign trail.
On Wednesday, Dick Harpootlian, a former chairman of the South Carolina Democratic Party and a prominent Obama supporter, said some of Clinton's recent remarks on the campaign trail were appeals based on race and gender, meant to "suppress the vote, demoralize voters and distort the record."
The Clinton campaign, however, says Bill Clinton is simply defending his wife's record and the Obama campaign is showing its frustration when it goes after him.
While it is unlikely Edwards would stage a come-from-behind victory, he still could have a significant impact on the primary process, which is increasingly coming down to a delegate race. Edwards can win delegates even if he does not win states.
On January 15, Edwards pledged, "I'm in this for the long haul. We're continuing to accumulate delegates. There's actually a very narrow margin between Sen. Obama, Sen. Clinton and myself on delegates."
Playing off the recent bickering between the Obama and Clinton camps, Edwards Friday launched an ad that highlights the heated back-and-forth between the two rivals.
The ad, called "Grown-up," is Edwards' latest effort to draw attention to the ongoing scuffle between his rivals, while painting himself as above the fray.
The ad echoes comments Edwards has repeatedly made on the trail since the debate -- that he represents "the grown-up wing of the Democratic Party." E-mail to a friend
CNN's Peter Hamby, Alexander Mooney, Bill Schneider, Suzanne Malveaux and Jessica Yellin contributed to this report.