COLUMBIA, S.C. — Arizona Sen. John McCain won the pivotal South Carolina primary yesterday as both the Republican and Democratic presidential contests moved forward with some clarity but still much uncertainty.

Suddenly Florida, where one-time GOP front-runner Rudy Giuliani decided to make what may be a first or last stand, becomes an extremely important crossroad Jan. 29 for the Republicans. It will be the last single-state test before more than 20 states hold contests Feb. 5.

McCain's victory over ex-Arkansas Gov. Mike Huckabee came on the same day Hillary Rodham Clinton won what may have been a momentum-building contest over fellow Sen. Barack Obama of Illinois in Nevada's Democratic caucuses. The Democrats now turn to South Carolina for the Jan. 26 primary and what likely will be the first real test of black voters' loyalties.

Yesterday also included another less significant GOP triumph. Former Massachusetts Gov. Mitt Romney, who was holding onto a distant third in South Carolina, won a largely uncontested Nevada caucus. Rep. Ron Paul, R-Texas, struggled with McCain for a distant second in Nevada.

Romney now has three wins but only one in a state where he had major competition, Michigan. He also won the Wyoming caucuses eight days ago.

South Carolina is a traditional pivot point in GOP contests, and eight years ago it crushed McCain's bid for the Republican nomination.

McCain's victory here was a statement. It proved he could win in deep-red GOP territory and defeat a Southern governor in the process.

The Vietnam veteran ran largely a biographical campaign here, highlighting his experience and judgment while the folksy Huckabee continued his populist call for change.

With his win in New Hampshire less than two weeks ago, McCain has won two of the four most fiercely contested states, giving him momentum leading into another multicandidate scramble in Florida in nine days.

But momentum has been a fickle commodity in 2008. Romney won Michigan on Tuesday but stumbled badly here and pulled out to pick off Nevada. If McCain loses in Florida to the former New York Mayor Giuliani or other GOP contenders, the Republican race could approach voting on Feb. 5 as muddled as ever.

South Carolinians voted on a dreary day of cold rain and chilly news. State officials declared Friday that unemployment in the state had risen to 6.6 percent, the largest jump in 20 years. The issues debate here had turned sharply toward economics, a trend that is likely to continue as Democratic presidential candidates arrive here for their own South Carolina showdown Saturday.

Huckabee's close second keeps him in the fight. But a major rationale for his candidacy — that he is a Washington outsider who comes from the GOP's strongest regional base — was damaged in the first Southern primary.

His failure to win in South Carolina, where nearly half the voters told exit pollsters they were born-again Christians, does not bode well for bigger states like New York, Illinois and California that will vote on Feb. 5 and where the percentage of evangelical voters is likely to be much lower.

Iowa, won by Huckabee, and New Hampshire, won by McCain, are true swing states in the general election. Michigan, won by Romney, has shifted decisively Democratic since 1992.

But South Carolina remains one of the most Republican states in the country, the kind of state that the maverick McCain had trouble winning in his first presidential run eight years ago.

"Whoever wins in South Carolina gets a ripple effect, that you can really carry red states," said Clemson University political scientist Dave Woodard. "That is why we are important."

McCain's victory here may already have knocked out one rival, former Sen. Fred Thompson of Tennessee, who was in a struggle for third. In a post-election address here Thursday, he did not indicate his plans specifically but referred to his candidacy in the past tense.

"Stay strong," he told supporters.

In a risky strategy, Giuliani has largely bypassed early contests for what could be a first or last stand in Florida. His campaign has gone into a January deep freeze while others picked up victories and momentum. McCain could build on the momentum with a victory in the Sunshine State, or the GOP contest could be thrown into more uncertainty if Giuliani or someone else wins.

"I really don't think South Carolina has delivered the winner like it did in the past," Woodard said. "But if (McCain) beats Giuliani (in Florida) that really sets him up for Feb. 5."