Pat Dollard | Young Americans

COLUMBIA, South Carolina (Reuters) - Democrat Barack Obama expanded his lead on rival Hillary Clinton to 15 points heading into South Carolina’s bitterly contested presidential primary, according to a Reuters/C-SPAN/Zogby poll released on Saturday.

Obama, an Illinois senator, gained two points on Clinton overnight to lead 41 percent to 26 percent just hours before voting began in Saturday’s primary. John Edwards was in third place after slipping two points to 19 percent.

In Florida, where Republican presidential contenders meet in a critical primary on Tuesday, John McCain had a narrow 3-point advantage on rival Mitt Romney, 31 percent to 28 percent, in the state’s initial rolling poll.

The polls in both states had a margin of error of 3.4 percentage points.

Obama has led Clinton by double-digits in all four days of polls in South Carolina, fueled by a huge advantage among the black voters who are expected to make up about half of the electorate in the first Democratic primary in the South.

Obama, who would be the first black U.S. president, was favored by 62 percent of black voters, with Clinton at 18 percent and Edwards at 5 percent.

Edwards, a former North Carolina senator who won the state during his failed 2004 White House run, and Clinton, a New York senator, were tied among white voters at 35 percent each. Obama was at 19 percent.

“Obama holds solid leads in every section of the state, and among both men and women,” pollster John Zogby said.

Clinton and Obama have clashed fiercely during the last week over their records in an increasingly rancorous duel for the right to represent the Democratic Party in November’s election.

South Carolina is the next test in their back-and-forth battle for the Democratic nomination. Obama won the first contest in Iowa but Clinton won the next two in New Hampshire and Nevada.

While Obama spent the week in the state campaigning, Clinton left for two days and will be gone again when the results are announced. About 10 percent of Democratic voters remain undecided.

McCAIN VS. ROMNEY

Florida’s Republican primary matches McCain, an Arizona senator, with Romney, a former Massachusetts governor, after they split two contests last week — McCain won South Carolina’s Republican primary and Romney won in Michigan.

The winner in Florida will gain valuable momentum heading into the February 5 “Super Tuesday” voting, when 22 states will have either Republican or Democratic nominating contests.

Trailing the top two Republican contenders was former New York Mayor Rudy Giuliani, who once led national polls in the race but has seen his standing plummet as he bypassed the early voting states to concentrate on Florida.

Giuliani was in third place at 15 percent, with former Arkansas Gov. Mike Huckabee, the winner in Iowa, at 10 percent. Texas Rep. Ron Paul had 5 percent.

Romney led easily among those voters who identified themselves as “very” conservative, while McCain held a 2-to-1 edge among moderates.

Religious conservatives fueled the Iowa win by Huckabee, an ordained Baptist minister, but he trails both McCain and Romney in Florida among those who call themselves “born again.”

Nine percent of Republican voters in Florida remain undecided.

“It’s very close,” Zogby said. “With nearly one in every 10 Republicans undecided, there is still a long way to go in this one.”

The poll in Florida, as in South Carolina, showed the economy was the top issue among likely voters, at 38 percent. In Florida, the war on terrorism ranked second at 14 percent, ahead of the war in Iraq at 12 percent.

Democrats also will hold a primary in Florida, but because of a dispute between the state and national parties over the date of the contest none of the presidential contenders have campaigned there.

The rolling poll of 816 likely Democratic voters in South Carolina was taken Thursday and Friday. The Florida poll of 814 likely Republican voters was taken Wednesday through Friday.

In a rolling poll, the most recent day’s results are added while the oldest day’s results are dropped in order to track changing momentum.