Donald Trump has won South Carolina’s Republican primary, easily beating former United Nations Ambassador Nikki Haley in her home state.

Mr Trump’s victory means he has inched closer to a third straight nomination with the former president cleaning up at every contest that counted for Republican delegates.

His latest victory will increase pressure on Ms Haley to drop out of the race, making an election rematch between Mr Trump and President Joe Biden increasingly inevitable.

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Former President Donald Trump attends a primary election night party at the South Carolina State Fairgrounds (Andrew Harnik/AP)

“I have never seen the Republican Party so unified as it is right now,” Mr Trump declared, taking the stage for his victory speech mere moments after polls closed.

“You can celebrate for about 15 minutes, but then we have to get back to work.”

Mrs Haley was unable to dent Mr Trump’s momentum in her home state, despite holding far more campaign events and arguing that the indictments against the former president will hinder his challenge for the White House.

She has vowed to stay in the race through at least the batch of primaries on March 5, known as Super Tuesday.

The Associated Press declared Mr Trump the winner as polls closed statewide at 7pm.