Trump refuses to back any GOP nominee but himself

US Republican front-runner Donald Trump has dropped his pledge to support the party candidate if he does not win the nomination for the November election.

Trump's rivals, Ted Cruz and John Kasich, refused to say outright whether they would unite behind a nominee. All the Republican candidates signed a loyalty pledge last November.

It is the latest sign of friction between Trump and Cruz, who have been embroiled in a dispute involving each other's wives.

A committee supporting Cruz published a nude photo of Trump's wife Melania from 2000. In retaliation, Trump tweeted an unflattering picture of Cruz's wife Heidi.

Asked by CNN about the dispute, Trump said: "He started it."

CNN presenter Anderson Cooper said: "But, sir, with all due respect, that's the argument of a five-year-old."

"No, it's not," said Trump. Trump claims he has been "treated very unfairly" in the campaign. To a question about whether he maintained his loyalty pledge from last November, Trump said: "No, I do not any more."

Earlier, when asked the same question, Cruz did not give a direct answer but said: "I am not in the habit of supporting someone who attacks my wife and attacks my family."

The billionaire has been repeatedly criticised by his rivals and observers for a campaign that has included personal attacks at rivals and criticism and violence against protesters.

Earlier on Tuesday, Trump's campaign manager Corey Lewandowski was charged with assaulting a journalist at a campaign event. Police said Lewandowski grabbed former Breitbart reporter Michelle Fields' arm when she tried to ask Trump a question, leaving a bruise.

Despite the numerous recent controversies surrounding him, Trump is currently well ahead in the Republican race with 739 delegates to Cruz's 465.

Kasich is some way behind with 143, with the 1,237 needed to win the nomination probably out of his reach.

The three candidates have campaigned in Wisconsin ahead of next Tuesday's primary there.