The final tracking poll of 604 likely voters conducted by Zogby International on Thursday and Friday found Bush leading McCain by 47-39 percent -- a gap on the outside edge of the survey's statistical margin of error of plus or minus 4 points. Talk radio host Alan Keyes polled 7 percent and the remaining 7 percent were undecided.

When these were asked which way they leaned, Bush widened his lead further to 51 to 42 percent with 7 percent for Keyes, which pollster John Zogby said was his final projection.

"It looks like Bush has it. Obviously anything can happen but McCain's only hope is a truly massive, unprecedented turnout of non-Republican voters," said Zogby.

The poll showed Bush picking up last-minute momentum as the campaign entered its closing days. In Thursday's poll, Bush led 43-42 percent. In Friday morning's survey, he led 43-40 percent. McCain has never led in any poll taken in the state in the past week.

Usually, undecided voters tend to side with the underdog in close races but in this case they are breaking for Bush.

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

The primary has become a crucial test in the battle for the Republican presidential nomination but the race has turned out to be 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.

Within the poll sample, 59 percent were Republicans, 26 percent were independents and 15 percent were Democrats.

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.