The poll of 606 likely voters conducted by Zogby International on Wednesday and Thursday found Bush leading McCain by 43-40 percent -- a gap well within the survey's statistical margin of error of plus or minus 4 points.

Talk radio host Alan Keyes polled 5 percent and the remaining 12 percent were undecided. When these were asked which way they leaned, 21 percent said McCain, 17 percent said Bush and the rest still declined to state a preference.

In Thursday's poll, Bush led 43-42 percent. Bush's support has held steady at 43 percent since the poll began on Wednesday, while McCain's has fluctuated slightly, depending on the number of independent voters and Democrats who said they intended to participate in the primary.

"Bush has a small but steady lead but it could disappear if there is a very high turnout in the primary, as there was in New Hampshire," pollster John Zogby said.

McCain won the New Hampshire primary on Feb. 1 by 19 points, far exceeded pollsters' predictions, due to a huge turnout of independent voters.

Because the race is so close, Reuters will release one final poll on Friday at 10 p.m. EST, bringing in a further 300 interviews conducted throughout the day and evening.

The primary has become a crucial test in the battle for the Republican presidential nomination but the race has turned out to be especially hard to predict because South Carolina's open primary system allows independent voters and Democrats to vote.

McCain himself said turnout was the key factor in the race. He said he would win if turnout was high and would probably lose if it was low.

In the poll so far, Bush beats McCain among Republicans by almost two to one while McCain sweeps Bush among Democrats and independents.

Within the poll sample, 61 percent were Republicans, 26 percent were independents and 13 percent were Democrats.

Some 40 percent said this would be the first Republican primary in which they would participate while 28 percent said they had voted in past Democratic primaries and 25 percent said they voted for President Clinton when he was the Democratic nominee in the 1996 presidential election.

When they were asked which Republican had the best chance of winning the 2000 election, 50 percent said Bush and 37 percent said McCain.

Bush also had a higher favorable rating in the poll. He was rated positively by 80 percent and unfavorably by 16 percent. McCain's positive rating was 77 percent while 16 percent viewed him unfavorably.

The Texas governor has been hammering McCain over the air waves in the past two weeks, accusing him of being a deserter from traditional Republican beliefs in an effort to shore up support among the Republican base.

McCain stopped his negative advertisements last week and has been running what he calls an exclusively positive campaign.