Billionaire Donald Trump romped to victory in the South Carolina primary election as former Florida governor Jeb Bush bowed out, in a night that encapsulated the topsy-turvy nature of the Republican Party's campaign for the November presidential poll so far.

Just six months ago, Mr Bush was considered a firm favourite for the party's nomination as presidential candidate while Mr Trump was regarded as nothing but an entertaining sideshow.

Since then, a wave of anti-establishment sentiment among party members has propelled the outspoken businessman to the top of the popularity ratings while the highly qualified, well-funded party favourite has failed to gain traction.

A last-gasp attempt by Mr Bush to deploy his brother, former president George W. Bush, to help rally voters in South Carolina yielded nothing.

Mr Jeb Bush ended last Saturday night in fourth place, with just 7.8 per cent of the votes, behind a dominant Mr Trump (32.5 per cent) and two senators effectively tied for second place, Mr Marco Rubio (22.5 per cent) and Mr Ted Cruz (22.3 per cent).

"The people of Iowa, New Hampshire and South Carolina have spoken. And I really respect their decision. So tonight, I am suspending my campaign," said a clearly emotional Mr Bush, referring also to two earlier primaries. His supporters yelled "No! No!" in response.

Mr Bush's departure - the highest profile exit of the race so far - leaves the once-crowded Republican field looking like a three-man race.

For all the optimism of fifth-placed Ohio Governor John Kasich and last-placed surgeon Ben Carson, the top three finishers are so far ahead that they appear to be the only viable candidates.

Even Mr Rubio decided to declare the narrowing of the field in his speech after the South Carolina results were announced.

"After tonight this has become a three-person race, and we will win the nomination," he said.

The trio of Mr Trump, Mr Rubio and Mr Cruz present Republican voters with three distinct choices. Mr Trump is the political outsider representing those tired of Washington politics, Mr Rubio is now the clear establishment pick while Mr Cruz has positioned himself as the most conservative of the three.

Last Saturday, with the ethnic Indian Governor of South Carolina, Ms Nikki Haley, by his side, Mr Rubio spoke about a modern conservative movement that was more racially and economically diverse than the current Republican Party.

"The 21st century conservative movement is the daughter of immigrants from India, who wanted desperately for their children to have all the opportunities they never did, who faced a string of prejudice. Yet, because of the greatness of our country, Nikki Haley is today the governor of a state where it is always a great day," he said.

Of the top three finishers, it is perhaps Mr Cruz who was the most disappointed.

Nearly three-quarters of the state's Republican voters identify themselves as conservative evangelical Christians. The group drove Mr Cruz to victory in Iowa but a third-place finish in South Carolina made it clear he does not have a firm lock on them and is not as strong in the conservative south as initially thought.

"Cruz should not be in a tight race for second here. That's not a good sign for him," said political science professor Bruce Ransom of South Carolina's Clemson University.

As for Mr Trump, the ability to build on his big win in New Hampshire means that he is now the legitimate front runner for the nomination as the contests move on to Nevada and a string of southern states.

Dr Ransom noted that the South Carolina primary "has picked the correct Republican nominee at every election since 1980 except the last one in 2012".

"It may be too soon to say that Mr Trump has wrapped it up but he is certainly the favourite now."