COLUMBIA (AFP) - United States Democratic presidential hopeful Hillary Clinton defeated rival Bernie Sanders by an overwhelming margin in Saturday's Democratic primary in South Carolina, projections showed, seizing momentum ahead of the most important day of the nomination race: next week's "Super Tuesday" contests.

Four weeks into the White House primaries, the former secretary of state earned a decisive win in the first southern state to vote for a 2016 Democratic nominee, before the race broadens to 11 contests across the country.

"To South Carolina, to the volunteers at the heart of our campaign, to the supporters who power it: thank you," Mrs Clinton said on Twitter shortly after the projections were made.

"Tomorrow this campaign goes national," Mrs Clinton said to a loud roar as she thanked supporters in Columbia, South Carolina. "We are going to compete for every vote in every state. We are not taking anything, and we are not taking anyone, for granted."

US networks called the race for Mrs Clinton moments after polls closed in the Palmetto State, where the majority of Democratic voters are African-American, a voting bloc that she has successfully courted for decades.

Mrs Clinton also looked beyond her battle with Sen Sanders, tweaking the man many now see as the likely Republican nominee: Mr Donald Trump, whose campaign slogan is "Make America Great Again".

"Despite what you hear, we don't need to make America great again. America has never stopped being great," she said. "But we do need to make America whole again," she added, laying out an argument against the divisive rhetoric favoured by Mr Trump, who has antagonised immigrants, Muslims and campaign rivals.

"Instead of building walls, we need to tear down barriers," Mrs Clinton said. "I know it sometimes seems a little odd for someone running for president these days and in this time to say we need more love and kindness in America," she added. "But I am telling you from the bottom of my heart, we do."

With nearly 79 per cent of precincts reporting, Mrs Clinton stood at 73.6 per cent compared with 25.8 per cent for Sen Sanders.

The win marks a moment of redemption for Mrs Clinton, who in 2008 lost badly in the state to current President Barack Obama, whose victory here served as a turning point for his own campaign.

Exit polls in South Carolina showed African-American voters - who represented 61 per cent of all Democratic voters in the primary - backed Mrs Clinton by a stunning 87 per cent, against 13 per cent for Sen Sanders.

Saturday's result was a foregone conclusion of sorts, with Sen Sanders bolting from the state on Saturday morning to head to Texas and then Minnesota, two states in play next Tuesday when Sen Sanders is aiming to keep his head above water if he wants to challenge Mrs Clinton deeper into the nomination race.

Sen Sanders swiftly offered his congratulations, but insisted he was in it for the long haul.

"Let me be clear on one thing tonight. This campaign is just beginning," he said in a statement.

"We won a decisive victory in New Hampshire. She won a decisive victory in South Carolina. Now it's on to Super Tuesday."

Among Democrats, Mrs Clinton leads in the national delegate count at this early stage, having now won three of the first four nomination contests - in Iowa, narrowly, then Nevada and South Carolina.

As the Democrats voted, the Republican churned on with frontrunner Donald Trump trading barbs with rival Marco Rubio, who in recent days has launched a fierce assault on the billionaire real estate mogul.

"I want to save the (Republican) party from a con artist," Sen Rubio, seen by many as the man best-positioned to topple Mr Trump, said at a stop in Kennesaw, Georgia.

Sen Rubio accosted Mr Trump for "flying around on hair force one," and having "the worst spray tan in America."

Mr Trump pushed back during a speech in Arkansas, one of the dozen states voting Tuesday.

"I watched this lightweight Rubio, total lightweight, little mouth on him, bing, bing, bing," Mr Trump sneered. "I'm a con man, right? I built a great business!"

The brawling among Republicans may reinforce Mrs Clinton's argument that she has the temperament and experience to be commander in chief.

"When you are president, the entire world listens to every word you say," Mrs Clinton said in Birmingham, Alabama, in an apparent attack on Mr Trump's badgering of rival candidates and his caustic remarks about immigrants and Muslims.

Sen Sanders, a self-declared democratic socialist who represents Vermont in the US Senate, headed earlier Saturday to Texas, where he told some 10,000 people that he has been this election's comeback kid.

Sen Sanders is also focusing on states like Ohio and Minnesota that vote later in March, when a whopping 45 percent of the delegates who will attend the nominating convention are up for grabs.

Only 3 per cent of delegates for July's nominating convention in Philadelphia will have been awarded by Saturday's end.

But the 11 states that hold Democratic nominating contests next Tuesday will send a whopping 18 per cent of the delegates to Philadelphia.

Mrs Clinton is ahead in most, but Sen Sanders has the edge in Massachusetts and his home turf of Vermont.

Sen Sanders, who calls himself a democratic socialist, told a crowd of some 10,000 people earlier in Texas that he has been this election's comeback kid.

Looking beyond next Tuesday, he is also focusing on states like Ohio and Minnesota that vote later in the crucial month of March, during which a whopping 45 per cent of all delegates to the nominating convention are up for grabs.

Retired teacher Elvira Kennedy, 70, voted for Mrs Clinton in Columbia.

"She's the best candidate," said Ms Kennedy, who is African-American.

"We never had a woman president, it's about time we give a woman a chance to mess everything. Men have been doing it for 300 years."