CHARLESTON, South Carolina (Reuters) - Barack Obama has a 13-point lead on rival Hillary Clinton but his support has eroded slightly on the eve of South Carolina's Democratic presidential primary, according to a Reuters/C-SPAN/Zogby poll released on Friday.
Obama's edge on Clinton slipped by two points overnight but remained in double digits, 38 percent to 25 percent, in the rolling poll, with John Edwards gaining two points to climb to 21 percent and inch closer to second place.
The poll has a margin of error of 3.4 percentage points.
Obama, an Illinois senator who would be the first black U.S. president, had an 18-point lead over Clinton in the initial poll published three days ago but has seen his support fall from 43 percent to 38 percent.
Obama and Clinton have battled fiercely over the last week, trading accusations about their records in an increasingly rancorous duel for the right to represent the Democratic Party in November's election.
Obama continues to enjoy strong backing among black voters, who are expected to be more than half of the electorate in Saturday's Democratic primary. Obama won 55 percent of blacks in the poll, with Clinton at 18 percent.
"Obama still leads, but the lead keeps going down," pollster John Zogby said.
In the last day of polling on Thursday, Obama's advantage over Clinton, a New York senator who would be the first woman U.S. president, was only 7 percentage points.
Edwards, a former North Carolina senator who won South Carolina during his failed 2004 presidential bid, has climbed steadily each day of the poll. He started at 15 percent and is now at 21 percent.
Edwards held a slight lead over Clinton among likely white voters at 36 percent to 33 percent. Obama had 18 percent.
"Edwards is in a battle for second place," Zogby said. "He's getting the lion's share of the undecideds."
The number of likely Democratic voters who said they were uncertain of their choice dropped two points to 11 percent, a still sizable figure one day before the voting.
South Carolina is the next test in the 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.
Obama has offered pointed criticism of not only Clinton but also her husband, former President Bill Clinton, who has criticized Obama and called the idea of his consistent opposition to the Iraq war "a fairy tale."
But the attacks have not dimmed admiration for Bill Clinton among the party faithful, with 75 percent saying they had a favorable view of him and only 20 percent holding an unfavorable view.
The rolling poll of 811 likely Democratic voters in South Carolina was taken Tuesday through Thursday. 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.
(Editing by Jackie Frank)
For more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at blogs.reuters.com/trail08/