"The establishment — they want to throw that right out the window. Because if I get out, all those people are going, they're all going with me," Trump said on MSNBC's "Morning Joe.'
Pressed about whether he was still willing to run as an independent, Trump said, "Well, look. Do I want to? No. I'm leading by a lot [among] Republicans."
"But they're spending now $100 million on negative, phony ads on Trump University," Trump said.
Pushed again on potentially launching a third-party bid if Republicans do not unite behind him, Trump referred to the letter he signed with the Republican National Committee (RNC) last year pledging to support the GOP.
"I signed a letter with the RNC and I said I want to do this as a Republican," Trump said. "But I'm not being treated the right way. I'm not being treated properly."
"Whether I ran as an independent or not, those people will never go out and vote ... they didn't vote for Romney last time," Trump said.
Republicans are unleashing last-ditch efforts to stop Trump, a who has racked up delegates with a series of state wins despite criticism from conservatives.
Mitt Romney, a Republican party elder who served as its nominee in 2012, is unleashing a blistering attack speech
on Trump at the University of Utah on Thursday, calling him a "phony" and a "fraud."
During his "Morning Joe" appearance, Trump slammed Romney ahead of the speech, pointing out that Romney took his endorsement in 2012 when running for president.
Trump said that Romney "begged" him for his endorsement and called the former Utah governor a "disaster" in the final weeks of the 2012 campaign. "It was almost like he was lost."
"The biggest story in all of politics isn't so much what's going on with Mitt Romney, it's the fact that the Republicans are gaining millions and millions of people because of me," Trump said on MSNBC.
"Millions of people are joining the GOP, and they're leaving the Democrats and they're leaving independents and they're joining the GOP," Trump said.
This report was updated at 8:26 a.m.