Bouncing back from an 18-point defeat in New Hampshire, Bush ran up a victory margin in the Palmetto State that exceeded all predictions and cast serious doubt on McCain's ability to recover.
Based on nearly complete returns, Bush won a clear majority of the vote, defeating McCain by a projected 11 percentage points. Alan Keyes was a distant third, with about 5 percent of the votes cast.
Key to Bush's success was an all-out push by his campaign to turn out conservative Republican voters, and in particular members of the religious right, who backed him by overwhelming margins.
Bush told supporters in the state capital of Columbia that he was "honored and humbled by the huge victory we had in South Carolina."
"Tonight, there are only 263 days more to the end of Clinton-Gore," he said to cheers.
In a TV interview last night, Bush said he "did a good job of uniting our party."
The Texan, who assumed a much more aggressive style here, said he did "a better job as a candidate" than in New Hampshire and brushed aside criticism of his negative campaigning.
"I stand by what I did," he said on CNN. "I defended myself."
McCain, however, left little doubt that he regarded Bush's bare-knuckle tactics as having been the decisive factor.
In scathing terms, McCain laced his concession speech with bitter references to his rival.
Saying he intended to "keep fighting clean," McCain told supporters in North Charleston that he "will not take the low road to the highest office in this land. I want the presidency in the best way, not the worst way."
He contrasted himself with Bush, suggesting that the Texas governor had let "ambition overcome principle." He linked Bush's campaign to "defeatist tactics of exclusion" by the GOP establishment that would lead to a Republican defeat in November.
McCain compared his record of reform to Bush's "empty slogan of reform" and portrayed the contest between himself and the son of the former president as a choice "between experience and pretense."
The fierce 18-day campaign in this state ended on a balmy day that helped draw record numbers of voters to polling places. By one estimate, the turnout may have reached 600,000, more than twice the previous record, set in the 1996 primary.
An unprecedented number of independents and Democrats, many of them lured by McCain's crossover appeal, cast votes in the Republican primary.
But Bush's "fire wall" in this state served its intended purpose, halting McCain's surging campaign behind a huge wave of votes from social and religious conservatives.
The size of Bush's victory once again makes him the front-runner in the Republican race and raises the stakes for Tuesday's primary showdown in Michigan.
McCain refused in a TV interview to concede that he must win Michigan to keep his nomination hopes alive. The latest polling shows the race there to be a dead heat.
"Michigan will be very important," the senator said. But "no matter what," McCain said, he will remain in the race at least until March 7, when 16 states, including Maryland, hold primaries or caucuses.
There was no Democratic contest in South Carolina yesterday, and the number of non-Republicans who cast ballots in the primary was unusually high. According to exit polls, about 40 percent of the total vote came from non-Republicans.
McCain claimed that he carried Charleston, the state's largest city and a Democratic stronghold. Slightly less than 10 percent of the vote was cast by Democrats, in line with pre-election estimates.
Bush took about one-third of the independent vote, exit polls showed, then rolled up lopsided margins among Republican voters.
According to Election Day interviews of voters leaving polling places around the state, Bush swept the votes of those who identified themselves as Republicans by a 68-26 percentage point margin.
"It's very difficult to construct a strategy of winning a Republican nomination when you're going to lose Republican voters by 40 points," said Whit Ayres, a Republican pollster who is neutral in the race, referring to McCain's failure to generate more support among Republicans.
Bush's support from religious conservatives, who favored him by a nearly 3-to-1 margin, was an essential element in his victory.
The Bush campaign's outreach to Christian conservatives was spearheaded by Ralph Reed, former executive director of the Christian Coalition and now a Republican consultant.
Using paid phone banks, volunteers recruited from individual churches, advertising on Christian radio stations and other means of communication, the Bush forces hammered McCain's positions on abortion, taxes and campaign reform. Among the lines of attack: harsh criticism of a provision in McCain's tax-cut plan that would eliminate much of the deduction for certain charitable contributions to churches and private schools.
McCain abandoned his attack ads in the final week of the race here, after airing a TV spot that accused Bush of being as untrustworthy as President Clinton. Long after the ad had been withdrawn, Bush continued to counter-attack by reminding voters of that negative ad.
Until the time the polls closed yesterday, Bush continued to press his rival with attack ads on the radio that painted McCain as a hypocrite on his signal issue of political reform.
"Bush was willing to go out and do what it took to win," said Randy Tate, former executive director of the Christian Coalition and now a spokesman for the Voter.com Web site. "He went on the offensive, and it didn't hurt him because it was focused on issues."
One measure of the potency of Bush's anti-McCain ads could be seen in exit-poll findings, which showed that McCain's campaign was viewed by voters as being more negative than Bush's.
Bush reshaped his campaign for this state after his Feb. 1 loss to McCain in New Hampshire.
Calling himself "a reformer with results," the Texan promoted his record on education, taxes and health care in his home state. At the same time, his assault on McCain shifted some of the focus away from the senator's life story -- one of the main sources of his popularity -- and aimed it instead on his position on issues of importance to social and economic conservatives.
The argument appeared to hit home with South Carolina voters. Exit polls showed that voters, by a 61-52 margin, considered Bush more of a reformer than McCain.
Bush had tried to establish a "fire wall" in South Carolina to protect himself from an early primary defeat in New Hampshire. Other Republican front-runners, including Bush's father in 1988 and 1992, and Bob Dole in 1996, had done so.
But after McCain's victory in New Hampshire, Bush found his early lead in this state erased. Both men campaigned here almost full time over the past two weeks, generating more excitement and interest than previous primary contests here.
McCain claimed last night that Bush had outspent him as much as 10-to-1 here. Because Bush is not accepting federal subsidies for his campaign, he is not subject to the same spending limits that McCain is.
But estimates of spending by the two sides suggest that Bush spent more than $5 million in South Carolina to McCain's $4 million.
Bush's campaign is now spending at the rate of $3 million a week nationally, depleting a campaign treasury that once totaled $70 million and now is less than $20 million.
Alarmed that Bush was spending at that clip, some of his financial backers had become increasingly worried about his chances, and one of his most prominent California supporters defected to McCain.
The fear in the Bush camp was that a defeat in South Carolina might lead to more defections and erase the biggest political advantage the Texan brought to the campaign: the sense that his nomination was inevitable and the belief of party leaders that he is the most electable Republican candidate.
Once the polls began turning his way late last week, Bush dismissed as "weak-kneed" those former backers and said he was glad to know now which of his supporters would wilt in the heat of battle.
George W. Bush 53%
John McCain 42%
Alan L. Keyes 5%
99% of precincts reporting