Donald Trump Tuesday ripped some of his opponents in the Republican primary for refusing to endorse him, saying "I don't know how they live with themselves."

"They signed these really, really powerful pledges," Trump told Newsradio 970 in Tampa Bay, Fla. "If you read them, you would've said, 'Wow, that was a really good lawyer that drew that up.'

"They didn't back — and, you know, I don't know how they live with themselves in one way, because they have to live with themselves," he said. "When they have to run sometime in the future, I think it would be very difficult for them."

Trump's comments were reported by CNN.

Only four of Trump's 16 primary challengers have not endorsed him: Sen. Lindsey Graham, Ohio Gov. John Kasich, former Florida Gov. Jeb Bush and former New York Gov. George Pataki.

Last year, candidates signed a pledge with Republican National Committee Chairman Reince Priebus saying they would back the eventual nominee.

Trump, who initially said that he would not honor the pledge before changing course, said he would have backed the eventual nominee if he had lost the primary.

"We had some people that signed pledges and they signed the pledge and it basically says that, 'I swear that if such and such gets in that we will back him,' and I would have had to back them." Trump told the radio station. "I would've done that.

"I wouldn't have thought about it."