Updated

Illinois Sen. Barack Obama leads the Democratic presidential field in South Carolina, while Fred Thompson is challenging Rudy Giuliani for the top spot among Republican contenders, according to a new poll.

Obama, who has drawn thousands of supporters to recent appearances in this early voting state, had the support of 34 percent of likely Democratic voters, compared to 25 percent for his nearest rival, New York Sen. Hillary Clinton. South Carolina native John Edwards, who won the primary here in 2004, garnered only 12 percent of support in the Mason-Dixon poll.

The poll was conducted June 13-15 among 329 likely Democratic and 432 likely Republican primary voters. The poll has a sampling error margin among Democrats of plus or minus 5.5 percentage points and of plus or minus 5 percentage points among Republicans.

Thompson, an actor and former U.S. senator from Tennessee, had the support of 25 percent of the respondents despite not yet formally declaring his candidacy. Former New York Mayor Rudy Giuliani was his closest competition, with 21 percent of support.

Former Massachusetts Gov. Mitt Romney, who has built a strong campaign organization here, had 11 percent. Arizona Sen. John McCain, who has garnered considerable endorsements from South Carolina GOP officials, earned only 7 percent.

The results are somewhat surprising because the Republican candidates have been visiting South Carolina for months, while Thompson — who plans to be in South Carolina next week — has yet to campaign here this primary season.

Nationally, Clinton was leading Obama in the Democratic race by 33 percent to 21 percent in an Associated Press-Ipsos poll conducted earlier this month. The same survey had Giuliani with 27 percent, McCain with 19 percent and Thompson with 17 percent.

A USA Today/Gallup Poll released on Monday showed almost identical results to the AP-Ipsos survey for both races. In early June, a USA Today/Gallup Poll had Clinton and Obama statistically tied. The Gallup Poll's editor in chief, Frank Newport, has said that earlier survey either picked up a short-term trend or included Democrats who leaned more toward Obama than the overall population.