"I backed Mitt Romney," Trump said in Maine. "He was begging for my endorsement. I could have said, 'Mitt, drop to your knees.' He would've dropped to his knees."
Trump called Romney a failed presidential candidate who ran a "horrible campaign," and repeatedly labeled him a "choke artist."
He also criticized Trump's successes as a businessman, saying he inherited his riches and didn't "create it."
“What ever happened to Trump Airlines? How about Trump University? And then there's Trump Magazine and Trump Vodka and Trump Steaks, and Trump Mortgage? A business genius he is not," Romney said.
Trump shot back at Romney, touting his business record, and denying the failures Romney brought up.
"He came out, it was very nasty," Trump said. "I mean, I thought he was a better person than that.
"He doesn't mention that fact that I built a city on the west side of Manhattan, that I built buildings all over Manhattan," he continued. 
"They don't want to talk about 92-story buildings all over the place. They don't want to talk about the Bank of America building in San Francisco."
He continued to tout Trump University, which he said has a 98 percent approval rating.
Trump mentioned reports that Romney was persuaded not run for president this cycle by former Florida Gov. Jeb Bush.
"Can you imagine? Jeb, Jeb sold him." Trump said. "Jeb, he's a good salesman. … He's a high energy salesman.
"But Mitt was afraid of Jeb," Trump said.
"But I'll tell you, the real reason he chickened out. It wasn't Jeb. It was me."
If Romney did decide to run, Trump said, Clinton would "destroy him." 
He added that he was targeting Romney from the start of the race because the GOP could not withstand another general election loss. 
"Nobody could've been nastier than me in getting him not to run by saying he's a choke artist," Trump said. 
"And I will say, the reason I did that ... I love our country too much," Trump said.
"He doesn't have what it takes to be president."
- Updated at 3:28 p.m.