Donald Trump has begun wearing a bulletproof vest following a string of threats and growing violence at rallies, according to sources close to his campaign.

The detail is one of a number of insights revealed by New York magazine in its latest edition.

"People who know Trump say they've never seen him so tired," wrote Gabriel Sherman, the reporter.

"Several months ago, he began wearing a bulletproof vest, two sources close to the campaign told me, which has added to his discomfort on the stump, leaving him sweaty and spent after events."


Trump currently leads the race to amass the most delegates before June's Republican convention in Cleveland to pick the party's nominee for president.

He faces a stiff test in Wisconsin's primary today, in a state where he trails his rival Ted Cruz in opinion polls.

Democrats Hillary Clinton and Bernie Sanders also face off in Wisconsin today.

Details of his body armour emerged after a string of rallies marked by angry protests and violent confrontations.

Last month a campaign event in Chicago was cancelled amid fights between demonstrators and supporters. Days later, Trump's Secret Service security detail had to leap to his aid when a protester tried to rush the podium at a rally in Dayton, Ohio.

And last year Joaquin "El Chapo" Guzman, the Mexican drug lord, reportedly put a US$100 million ($147.6 million) bounty on Trump's head.

Campaign insiders say he has been wearing the vest since at least February.

New York magazine also reveals details of another form of protection, which may explain Trump's long running feud with Fox News and Roger Ailes, its chief executive and Republican Party power broker.

It claims Trump came to Ailes' aid in 2014 when the TV executive was embroiled in a law suit with his former PR adviser, Brian Lewis, whom he fired over an unflattering biography, written by the author of the magazine piece.

"When Roger was having problems, he didn't call 97 people, he called me," said Trump.

Lewis claimed to have information that would destroy Ailes and his TV network.

Trump, who knew the lawyer retained by Lewis, helped broker a payoff, according to the magazine, but in so doing learned all about the potentially damaging revelations.

If true, it would explain why Fox News was slow to defend its own journalist, Megyn Kelly, when she was lambasted by Trump following a televised debate last year.

Even Kelly has said she expected more support from colleagues such as Bill O'Reilly on the station.

"I do wish that O'Reilly had defended me more in his interview with Trump. I would have defended him more,' she said in the April cover story of More magazine.