Billionaire Donald Trump has won South Carolina's Republican (GOP) primary, while former secretary of state Hillary Clinton has won Nevada's Democratic caucuses, according to exit polls. Trump and Clinton have now positioned themselves as front-runners from their respective parties to contest the US presidential election on 8 November.

According to NBC News exit polls, Clinton has beaten her challenger Vermont senator Bernie Sanders by 53% to 47%. "Some may have doubted us, but we never doubted each other," Clinton told supporters at the Caesars Palace hotel in Las Vegas.

In the Republican presidential race, Trump has defeated former Florida senator Marco Rubio, Texas senator Ted Cruz and former Florida governor Jeb Bush. In the exit polls, Trump was leading with 32.5%, while Rubio, Cruz and Bush were trailing with 22.5%, 22.3% and 7.8% respectively. Bush has dropped out of the GOP presidential race after his fourth-place finish.

"I'm proud of the campaign that we've run to unify our country and to advocate conservative solutions. The presidency is bigger than any one person. It's certainly bigger than any one candidate," an emotional Bush said. "I firmly believe the American people must entrust this office to someone who understands that whoever holds it is a servant not the master," Bush said.

Trump's win in South Carolina coupled with his victory in New Hampshire, a second-place finish in Iowa and the Palmetto state win - which was the first southern state contest - has shifted the momentum in his favour before 11 states, mainly southern states, hold Republican primaries or caucuses on 1 March.

"People (pundits) gave me no chance in South Carolina. Now it looks like a possible win. I would be happy with a one vote victory! (HOPE)," Trump tweeted shortly before the television networks declared him the winner.

"There's nothing easy about running for president," Trump had said at his victory rally in Spartanburg on Saturday (20 February) evening. "It's tough, it's nasty, it's mean, it's vicious. It's beautiful - when you win it's beautiful."

The Democrats' next primary will be held in South Carolina on 27 February, while Republicans will hold their caucuses in Nevada on 23 February.