A group of former contestants on Donald Trump's reality television show The Apprentice put their old boss in the hot seat on Friday, saying the US Republican front-runner has widened racial divisions and should not be president.

Trump's one-time admirers, most from racial minorities, urged the New York billionaire to tamp down his divisive rhetoric as he campaigns to succeed Democratic President Barack Obama in the November 8 election.

"We are all disappointed and in some ways shocked to see what is being spewed from Donald regarding his views on women, immigrants, and the list goes on," said Randal Pinkett, winner of the 2005 fourth season of the reality television show.

"We strongly condemn Donald's campaign of sexism, xenophobia, racism, violence and hate," he said at a news conference in Manhattan. Pinkett said Trump "is not worthy of the highest office of the land" and said there had been glimpses of those attitudes in private conversations and time spent off-screen with Trump during the making of the TV show.

Running for 14 seasons, The Apprentice gave Trump a national platform and his often blunt and unfiltered style helped make the show a major hit. The show featured groups of business-minded contestants vying for a titular apprenticeship in Trump's organisation. At its peak, nearly 21 million people watched the show.

Trump's proposals to ban Muslims from entering the United States and to build a wall at the Mexican border have drawn criticism even within his party. His campaign has been accused of tacitly encouraging violence at large and rowdy rallies where Trump supporters have at times clashed with protesters.

On Friday evening, as protests broke out at a rally in Hartford, Connecticut, Trump belted out his usual response: "Get them out of here!"

"I always say 'don't hurt that person, right?'" he said.

Pinkett told Reuters he had contacted former "apprentices" and said their effort was independent and timed to precede New York state's crucial primary election on Tuesday. At the news conference, Pinkett was joined by former Apprentice contestants Tara Dowdell and Kwame Jackson; another former contestant, Marshawn Evans Daniels, participated via video link.

"Failing wannabees"

Their efforts seemed unlikely to dent Trump's comfortable advantage in New York opinion polls against Republican rivals Ohio Governor John Kasich and Senator Ted Cruz of Texas.

Trump dismissed his former aspiring protégés on Friday as "failing wannabes out of hundreds of contestants".

"How quickly they forget. Nobody would know who they are if it weren't for me," he said in a statement. "They just want to get back into the limelight like they had when they were with Trump. Total dishonesty and disloyalty."

Trump pulled ahead of Cruz and Kasich this week in the national Reuters/Ipsos opinion poll. Among Republicans, 45 per cent support Trump, compared with 29 per cent for Cruz and 21 per cent for Kasich.

On the Democratic side, Hillary Clinton and Bernie Sanders are tied at 47 per cent. The poll had a credibility interval of 4.6 percentage points.

New York Post endorsement

In an apparent bid to establish a more presidential tone, Trump turned from his usual platform of Twitter to the opinion pages of The Wall Street Journal to denounce the Republican National Committee over a nomination process he said was rigged.

Friday's piece, along with an endorsement by tabloid newspaper the New York Post, signaled a possible detente with media magnate Rupert Murdoch, whose News Corp owns both newspapers. A News Corp spokesman declined to comment on the relationship between the two billionaires.

Murdoch took to Twitter last year to denounce Trump's comments that many illegal immigrants from Mexico were bringing crime to the United States, tweeting: "Trump wrong." The Journal in July called Trump a "catastrophe" in a withering editorial.

Trump on Wednesday met privately with Fox News Channel anchor Megyn Kelly after a feud that had lasted months. Another Murdoch-controlled company, 21st Century Fox, owns the channel.

NBC, the network that aired The Apprentice, cut ties with Trump last year after he described some Mexican immigrants as criminals and rapists. Trump's other well-known NBC venture, the Miss USA pageant and Miss Universe pageants, was also dropped from the line-up around that time.

Other Apprentice contestants have backed Trump, including actors Stephen Baldwin, Gary Busey and Lou Ferrigno, former basketball star Dennis Rodman and reality television star Jesse James.

Pinkett, however, in remarks directed at Trump, said: "I am calling, we are calling, for you to do better."