Arizona Senator John McCain picked up key momentum Saturday in his bid to become the Republican presidential nominee, but there was still no clear frontrunner in the crowded centre-right pack.

McCain picked up 33 per cent of the vote in the first state in the socially conservative South to weigh in on the presidential contenders, edging out former Arkansas governor Mike Huckabee, a Baptist preacher and favourite of many Evangelical Christians.

Long considered a maverick in his conservative party, McCain, 71, has not been a darling of the Republican establishment, and he lost South Carolina and the nomination to President George W Bush in 2000. Despite this year's win, he's still no heir apparent.

Former Massachusetts governor Mitt Romney, 60, hopes to see to that. He picked up a win earlier Saturday in the Nevada caucus after being the only major Republican candidate to appear there.

The less sought-after state brought Romney 18 delegates to the presidential nominating convention in September, just one fewer than McCain gained in South Carolina. And Romney still leads in the all-important delegate count that ultimately decides the party's nominee, though the overwhelming majority of delegates are yet to be decided.

There remains no consensus candidate who appeals to voters on all the key Republican issues - the economy, national defence and social issues. With so many viable candidates - McCain, Romney and Huckabee each have victories in the first five contests - the field could remain murky beyond the so-called Super Tuesday on February 5, when more than 20 states go to the polls in both major parties.

McCain's win Saturday would have been a shock just a few months ago, when his campaign was given up for dead by virtually all major political commentators. Surveys of Republicans showed him lagging the top contenders, he struggled to raise money, and his campaign was forced to slim down operations and cut staff.

If Saturday's results show anything, it's the unreliability of hasty predictions.

The day also saw Senator Hillary Clinton take the centre-left Democratic Party's Nevada caucus with 51 per cent, beating Senator Barack Obama at 45 per cent and former senator John Edwards with 4 per cent.

Forecasts that an endorsement by a union that represents many casino workers in gambling mecca Las Vegas would swing the state toward Obama also proved premature, as voters exercised their independence.

Seven of nine caucus sites set up inside casinos to allow shift workers to participate flouted expectations they would help Obama.

Clinton's campaign was widely declared to be in grave jeopardy after her loss to Obama on January 3 in Iowa, only for the former first lady to roar back ahead of the pack with an unpredicted win five days later in New Hampshire.

The lesson of both Clinton's survival and McCain's return from oblivion is that neither nomination fight has been decided.

Clinton's restored frontrunner status could dissipate again within a week, though, when South Carolina's Democrats have their turn. The state's January 26 primary is the first where African-American voters will have a decisive role.

While Clinton has the goodwill of black voters, among whom her husband, former president Bill Clinton, remains popular, Obama has already run the most viable presidential campaign ever by an African-American, creating a powerful lure for minorities and young voters.

McCain, for his part, is still far from inevitable. At best, he's the frontrunner to eventually become the frontrunner, but he must first win in Florida, where the Republican primary is January 29.

In Florida, Huckabee has a natural constituency in the socially conservative northern part of the state, while Romney has the campaign war chest to continue fighting. Even as the field is expected to thin with the looming departure of former senator Fred Thompson, former New York mayor Rudy Giuliani will for the first time become a factor in Florida.

A social liberal, Giuliani has staked his campaign on making a splash in the Sunshine State, after making a tactical decision to avoid the other January contests. His big-state strategy - intended to capitalize on his name recognition and moderate appeal - has been widely ridiculed by the same media commentators who counted out McCain and believed Clinton on the ropes.

But a strong showing in Florida would put Giuliani back into the mix for Super Tuesday next month. The absence of a clear frontrunner leaves the door open for yet another surprise.