US election: Trump steps back from Republican support pledge
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.
Mr 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 Mr Trump and Mr Cruz, who have been embroiled in a dispute involving each other's wives.
A committee supporting Mr Cruz published a nude photo of Mr Trump's wife Melania from 2000. In retaliation, Mr Trump tweeted an unflattering picture of Mr Cruz's wife Heidi.
Mr Trump claims he has been "treated very unfairly" by Republican party leaders - some have expressed disquiet or downright opposition to him winning the nomination.
To a question about whether he maintained his loyalty pledge from last November, Mr Trump said: "No, I do not any more."
At the time of the pledge, Trump had said: "I have no intention of changing my mind."
All three Republicans appeared at a CNN event on Tuesday night in Wisconsin, where voters will pick their presidential nominees next week.
When asked the same question about supporting the eventual nominee, Mr 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."
Mr Kasich said only that he would "see what happens".
The fact none of the three will commit to supporting the winner shows how bitter the race has become, says the BBC's Anthony Zurcher in Washington.
And it will be very hard to unite the party after the convention in July, when the nominee will be formally chosen, he adds.
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, Mr Trump's campaign manager Corey Lewandowski was charged with assaulting a journalist at a campaign event.
Despite the controversies, Mr Trump is currently well ahead in the Republican race with 739 delegates to Mr Cruz's 465.
In the Democratic race, Bernie Sanders will be hoping in Wisconsin to capitalise on some stunning recent wins to overhaul Hillary Clinton's lead.
More on the Trump campaign
The 40-year hurt - how Bruce Springsteen articulated the forces that underpin the rise of Trump
Trumpisms - 22 things that Trump believes
A civil war - Lifelong Republicans turned off by Trump