"I'm going to win here. You mark my words," Bush said, buoyed by polls showing him ahead or tied in the state's Republican primary. Holding out hope for a record turnout of Democrats and independents, McCain said Bush should apologize to voters for running "a low campaign."

McCain's team was considering increasing its resources in Virginia, site of a Feb. 29 primary, the last stop before March 7, when a dozen states conduct GOP contests that could determine the nominee.

After two weeks of intense campaigning, the South Carolina race boiled down to whether McCain's populist appeal and political momentum could survive the sheer force of Bush's organization. McCain won New Hampshire's leadoff primary, staggering the Texas governor's front-running campaign.

Both camps braced for primaries Tuesday in Michigan and Arizona, believing that the winner of South Carolina's 37-delegate primary would get a boost elsewhere.

Bush, who trails McCain in surveys in Michigan and the senator's home state of Arizona, is supported by the Republican governors in both states. Michigan Gov. John Engler had promised a secure "firewall" for the Texas governor, but he acknowledged that his own political enemies in both parties are taking out their anger on Bush.

Republican "governors are a big help to him," Engler said in an interview. "But he also gets our baggage."

Bush said a victory in South Carolina would quell doubts about his candidacy amid defections by a handful of supporters and rumors that many more will jump ship if he loses here. In tough times, Bush said he was learning "who my friends are" and would take note of "people who are playing their cards a little early."

"If there are some people getting weak-kneed, we need to know who they are in the heat of battle," he told ABC.

McCain continued to portray Bush as a negative campaigner, though the senator ran attack ads himself until last week and never has stopped upbraiding Bush on the stump.

At a noisy college rally, McCain said Bush owes "an apology to the people of South Carolina for running such a low campaign."

In a last-ditch drive to get supporters to vote, McCain held rallies along the Atlantic Coast, which is considered a strong area for him. Bush scoured the GOP strongholds north of the capital of Columbia. Conservative Alan Keyes, the third major GOP candidate, barely registers in polls.

"The establishment can't stop us," McCain said at a rally of 1,000 supporters in Hilton Head.

Pollsters said the race was too close to call in an environment ripe for surprise. Absentee ballots were cast in record numbers, suggesting a large turnout which most analysts suspected would favor McCain.

Yet two new polls suggested that McCain is no longer considered the race's top reformer, a sign that Bush has made gains in questioning his rival's integrity.

Like Michigan and Virginia, this state allows non-Republicans to vote in GOP primaries. McCain is courting Democrats, independents and habitual party-switchers, while Bush is focused on party regulars.

McCain and Bush have built mirror-image coalitions: The Texas governor is supported by six out of 10 Republican voters; McCain is backed by six out of 10 Democrats and independents, public and private polls show.

The figures suggest that McCain must persuade more non-Republicans than party regulars to vote in the GOP primary, swelling turnout in numbers that would exceed his stunning New Hampshire showing.

"It all comes down to turnout, and McCain has a high hurdle to overcome," said Dave Woodard, a GOP strategist and Clemson University professor.

The election was the topic of discussion Friday at Mulligan's Restaurant and Pub in this community of party-switchers northwest of Columbia. Nibbling on their lunches, Jennifer Slight, Jamie Jefferies, Gail Hendricks and Frank Monroe said they routinely cross party lines -- and they plan to vote Saturday.

The three women said they had anguished over their choices, but decided this week to support Bush because of his record in Texas and their respect for his father, the former president.

"He can tap into his Daddy's knowledge," said Slight, 26.

But Monroe, 61, a cabinetmaker, said McCain "already has that knowledge in his head."

Along with swing voters like the Newberry diners, undecided voters could determine the race. Polls indicate as many as one in 10 people haven't made their choice.

"I'll make up my mind when I walk into the polling booth," said Tom Ervin of Anderson, who attended a Bush rally.

Looking beyond Saturday, McCain's team was considering whether to begin airing TV ads in northern Virginia -- a hotbed of Democratic and independent voters. He is already on the air in the state's other markets, but can't make a serious play for the state's 56 delegates without northern Virginia.

Gov. James Gilmore is a Bush supporter and, like Engler, has promised to stop McCain's momentum. McCain's political team concedes its Virginia organization his thin, but aides believe that won't matter so much if he wins South Carolina, Michigan and Arizona.

One state to the north, Rep. Roscoe G. Bartlett is expected to endorse Bush on Monday in what amounts to his third choice for the Republican presidential nomination. Bartlett first endorsed former Vice President Dan Quayle, then shifted to Steve Forbes after Quayle dropped out. Forbes quit the race Feb. 9.