Donald Trump has begun wearing a bullet proof 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.”

Mr 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 on Tuesday, in a state where he trails his rival Ted Cruz in opinion polls.

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