Arizona Senator John McCain has won the Republican primary in South Carolina, narrowly beating former Arkansas governor Mike Huckabee.

With more than 93 per cent of the precincts reporting, McCain had 33 per cent of the vote and Huckabee hovered at 30 per cent.

"It took us a while, but what's eight years among friends?" McCain joked to his supporters, referring to his loss in 2000 to George Bush. "My friends we are well on our way tonight, and I feel very good."

Earlier in the day, Mitt Romney sailed to victory in the Republican caucuses in Nevada. But it's South Carolina that is seen as a key state in the race to the White House. Since 1980, the winning Republican candidate in the state has always gone on to win the party's presidential nomination.

Romney, the wealthy former Massachusetts governor, won in Michigan and Wyoming but focused on Nevada. Romney is a Mormon, and about 20 per cent of Nevada caucus-goers on Saturday were Mormon.

Speaking in Florida on Saturday night, he thanked his supporters and his wife for his win in Nevada. Maverick Republican Ron Paul appears to have edged out McCain to take second place.

Huckabee, a Southern Baptist minister who placed fourth in Nevada, had worked hard in an effort to capture the substantial evangelical vote in South Carolina. He had spoken out against abortion and gay marriage.

"That's what we need to do, is to amend the constitution so that it's in God's standards rather than try to change God's standards so it lines up with some contemporary view of how we treat each other and how we treat the family," he told one group.

McCain had alienated some groups in South Carolina by supporting funding for stem-cell research. However, he is a Vietnam War veteran and South Carolina has a high proportion of military vets.

Democrats

After what was described as a hard fought battle that included accusations of dirty politics and voter suppression, Hillary Clinton won the Nevada caucuses for the Democrats. Barack Obama took second place, and John Edwards placed third.

Despite his second place finish, Obama captured about the same number of delegates as Clinton. He is also expected to have a strong showing in South Carolina, which holds its Democratic primary on Jan. 26.

Clinton's win in Nevada was an important victory in a Democratic race that could remain tight in upcoming primaries.

"I guess this is how the West was won," said Clinton after her victory, which analysts said was aided by strong support from women and Hispanics.

The win in Nevada is the second for Clinton. Earlier this month, the New York senator won the primary in New Hampshire. That victory followed a loss to Obama at the Iowa caucuses.

Clinton won 51 per cent of the votes, while Obama lagged behind at 45 per cent. John Edwards remained a distant third, winning about 4 per cent of the votes.

Clinton's win came after last minute accusations by her husband, former U.S. president Bill Clinton, that some pro-Obama unions were forcing their members to vote the union line. Earlier in the week, Clinton denied that a lawsuit launched by his wife's union supporters was tantamount to disenfranchising some casino and hotel workers. Had the mainly pro-Clinton union leaders won their suit, it could have made it more difficult for some Obama supporters to vote in Saturday's caucuses.

On Friday, Clinton and Edwards both attacked fellow top-tier candidate Obama for favourably mentioning Ronald Reagan, the Republican president between 1981 and 1989.

"Ronald Reagan, the man who busted unions, the man who did everything in his power to destroy the organized labor movement, the man who created a tax structure that favored the richest Americans against middle class and working families, ... we know that Ronald Reagan is not an example of change for a presidential candidate running in the Democratic Party," Edwards said.

Clinton said she didn't consider it a better idea to privatize Social Security, eliminate the minimum wage, undercut health benefits, shut down the government or drive the country into debt.

Obama's camp noted that Clinton has mentioned Reagan as one of her 10 favourite presidents.

The three candidates have also squabbled about race and fought legal battles over holding nine caucuses at casinos to allow shift workers to participate.

Obama had the support from the Culinary Workers Union, which represents 60,000 workers in Las Vegas. However, that support was not able to push him to victory.

With a report from CTV's Joy Malbon and files from The Associated Press