Democrat Barack Obama holds a double-digit lead on rival Hillary Clinton three days before South Carolina's presidential primary, aided by a huge edge among black voters, according to a Reuters/C-SPAN/Zogby poll released on Wednesday.

Obama leads Clinton 43 percent to 25 percent in the rolling tracking poll, with John Edwards a distant third at 15 percent heading into Saturday's contest. The poll has a margin of error of 3.4 percentage points.

More than half of the Democratic primary voters in South Carolina are expected to be black. Obama, an Illinois senator who would be the first black U.S. president, leads among African-Americans by 65 percent to 16 percent.

Clinton and Edwards are virtually tied among white voters, with Clinton leading 33 percent to 32 percent, while Obama is third among whites at 18 percent. Likely black voters make up slightly more than half of the poll sample.

"Obama is doing very well among African-Americans but getting a decent share of the white vote in a three-way race," pollster John Zogby said.

Obama and Clinton, a New York senator who would be the first woman U.S. president, have engaged in an increasingly contentious duel for the Democratic nomination in November's election to succeed President George W. Bush.

The majority of the polling in the three-day survey was done before Monday night's angry debate in Myrtle Beach, where Obama and Clinton traded harsh accusations about their records and Edwards chastised the pair for their squabbling.

EDWARDS GAINS

In the one night of polling conducted after the debate, Edwards, a former North Carolina senator who won the South Carolina primary during his failed presidential bid in 2000, appeared to have made some slight gains, Zogby said.

"The question is, who does Edwards help and hurt? What impact does he have?" Zogby said.

He said the race in South Carolina still showed some fluidity. About 14 percent of voters in the state are undecided, and about 20 percent of voters backing a candidate say they could still change their mind.

The economy was listed as the top issue among South Carolina voters, by 49 percent, with the war in Iraq second at 24 percent.

Obama led consistently among almost all sub-groups, including men, women, liberals, conservatives, young voters, low-income voters, high-income voters and union households.

Clinton edged Obama out among the oldest voters, above age 70, usually her greatest strength. Edwards led Obama among Republicans.

Obama won the first nominating contest in Iowa, but Clinton bounced back with consecutive wins in New Hampshire and Nevada. After South Carolina, the Democratic race goes national heading into the February 5 "Super Tuesday" round of contests in 22 states.

The rolling poll of 811 likely Democratic voters in South Carolina was taken Sunday through Tuesday.

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.