Obama with 13-point lead in South Carolina: poll

By John Whitesides, Political Correspondent

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.