Presumptive US Republican presidential nominee Donald Trump has denounced Mitt Romney, the GOP nominee for the 2012 White House race, as "ungrateful" for refusing to support his candidacy.

"I believe I won for him, or helped him win, fives states that he was going to lose in the primaries. ... He was ungrateful," Trump said.  

"What happened is I was rough on Mitt because I didn't think they treated me properly. I helped him, really helped him. Gave him a lot of money. Helped him with robocalls. Every single robocall I made, he won that state. Every single speech I made, he won the state, in terms of the primaries,” the billionaire businessman said.

A Romney aide told The Hill newspaper last week that the former Massachusetts governor will skip the Republican National Convention in Cleveland in July.

Romney has been a virulent critic of Trump throughout his Republican primary campaign, imploring voters to reject his candidacy.

In March, he said the victory of Trump in the 2016 presidential election “will have profound consequences for the Republican Party and, more importantly, for the country."

“If we Republicans choose Donald Trump as our nominee, the prospects for a safe and prosperous future are greatly diminished,” he warned.

"Here's what I know: Donald Trump is a phony, a fraud," said Romney, who, according to American scholar and researcher Dr. Kevin Barrett, is an organized crime figure.

Romney said that "dishonesty is Donald Trump's hallmark," pointing to his "bullying, the greed, the showing off, the misogyny, the absurd third-grade theatrics."