Donald Trump has dropped his pledge to support the GOP candidate if he does not win the Republican nomination for the November election.
His 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 Donald Trump and Ted Cruz, who have been embroiled in a dispute involving each other’s wives.
A committee supporting Ted Cruz published a photo of Donald Trump’s wife Melania from 2000. In retaliation, Donald Trump tweeted an unflattering picture of Ted Cruz’s wife Heidi.
Asked by CNN about the dispute, Donald Trump said of Ted Cruz: “He started it.”
CNN presenter Anderson Cooper replied, said: “But, sir, with all due respect, that’s the argument of a five-year-old.”
“No, it’s not,” said Donald Trump.
Donald Trump claims he has been “treated very unfairly” in the campaign.
To a question about whether he maintained his loyalty pledge from last November, Donald Trump said: “No, I do not anymore.”
Earlier, when asked the same question, Ted 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.”
He went on: “Donald is not going to be the GOP nominee. We are going to beat him.”
Donald Trump later replied, saying he did not need Ted Cruz’s support: “I have tremendous support right now from the people.”
The New York tycoon has been repeatedly criticized by his rivals and observers for a campaign that has included personal attacks at rivals and criticism and violence against protesters.
Despite the numerous recent controversies surrounding him, Donald Trump is currently well ahead in the Republican race with 739 delegates to Ted Cruz’s 465.
John 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 April 5 primary there.