COLUMBIA, S.C. -- Texas Gov. George W. Bush is locked in a virtual dead heat with Arizona Sen. John McCain among voters likely to cast ballots in Saturday's crucial Republican primary in South Carolina, according to a Reuters poll published Thursday.

The poll of 606 likely voters conducted by Zogby International found Bush leading McCain by less than one percentage point -- a statistically insignificant difference in a survey that had a margin of error of plus or minus 4 percentage points.

Rounding up the percentages, Bush had 43 percent, McCain 42 and talk radio host Alan Keyes polled 4 percent. The remaining 11 percent were undecided.

Among Republican voters, Bush led by a margin of 55 to 32 percent but McCain was ahead among independents 63 to 21 percent and among Democrats 56 to 20 percent.

"The key factor remains, how many independents and Democrats actually vote," said pollster John Zogby. "McCain is heavily dependent on Democrats and independents while Bush has consolidated his hold among core Republican groups."

The survey was conducted Tuesday and Wednesday and was the second in a series of tracking polls being taken by Reuters in the run-up to Saturday's South Carolina primary.

In the poll released Wednesday, Bush had led McCain 43 to 40 percent. But the nightly results have found considerable volatility in the race.

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.

Reuters will publish a new poll Friday, based on a two-day rolling sample of about 600 voters.

Within the poll sample, 64 percent were Republicans, 20 percent were independents and 16 percent were Democrats.

Some 37 percent said this would be the first Republican primary in which they would participate while 26 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 39 percent said McCain, a slight decline for Bush and rise for McCain over the past 24 hours.

Bush also had a higher favorable rating in the poll. He was rated positively by 80 percent and unfavorably by 17 percent. McCain's positive rating was 77 percent while 13 percent viewed him unfavorably.

Half the poll was taken after a debate Tuesday evening in which Bush and McCain bitterly attacked one another.

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