Nikki Haley, the former South Carolina governor and US ambassador to the United Nations, has dropped out of the Presidential race after a prolonged fight as the last remaining challenger to Donald Trump for the Republican Party's presidential nomination.

"I said I wanted Americans to have their voices heard. I have done that," Ms Haley said in her concession speech. "I have no regrets. And while I will no longer be a candidate, I will not stop using my voice for the things I believe in."

Ms Haley's decision to drop out comes after suffering 14 state primary defeats in a matter of hours on Super Tuesday. Her only victory, the Vermont GOP primary, marked her second-ever win this cycle after securing Washington, DC on Sunday. Notably, the former ambassador did not endorse Mr Trump after backing down.

Now, Mr Trump stands alone as the GOP-frontrunner.

The former president, Donald Trump will presumably continue to rack up state primary wins until the Republican National Convention (RNC), scheduled for this July.

Prior to Super Tuesday, Ms Haley finished third in the Iowa caucuses and went on to come second in New Hampshire, picking up 43.2 per cent of the vote to Mr Trump's 54.3 per cent, prompting the frontrunner to express his irritation after she delivered an upbeat address to her supporters rather than admit defeat and bow out.

She also suffered the humiliation of scoring fewer votes than the "none of these candidates" box on Nevada ballot papers and lost overwhelmingly to Mr Trump in her own backyard of South Carolina as well as in Idaho, Michigan and Missouri and in North Dakota.

The former president has targeted her from day one, from labelling her with belittling nicknames ("Birdbrain", "Tricky Nikki") to threatening her donors with ex-communication from his movement and taunting her over her husband's absence from the campaign trail (Major Michael Haley is deployed overseas with the South Carolina Army National Guard).

There, the GOP delegates from each state will vote to nominate the Republican Presidential candidate - a victory for Mr Trump all but guaranteed at this point in the race.