Story highlights Donald Trump slammed Mitt Romney for criticizing him Thursday after asking for his endorsement 4 years ago

The brash billionaire said Romney "would have dropped to his knees" for his support in 2012.

Romney said earlier Thursday that Trump does not have the temperament to be president and that his party is at a crossroads

Washington (CNN) Donald Trump struck back at 2012 GOP presidential nominee Mitt Romney Thursday, blasting the former Massachusetts governor for "begging" for his endorsement four years ago only to sharply criticize him now.

"I don't know what happened to him," Trump said during a rally in Portland, Maine. "You can see how loyal he is. He was begging for my endorsement. I could have said, 'Mitt, drop to your knees.' He would have dropped to his knees."

Just hours before Trump, Romney gave a speech blasting the reality TV star as bad on foreign policy and lacking the temperament to be president.

"Think of Donald Trump's personal qualities, the bullying, the greed, the showing off, the misogyny, the absurd third grade theatrics," he said. "We have long referred to him as 'The Donald.' He is the only person in America to whom we have added an article before his name. It wasn't because he had attributes we admired."

But Garrett Jackson, a former personal aide to Romney, later tweeted on Thursday that the former Massachusetts governor never "begged" Trump for his endorsement -- adding that he wished "I would've recorded Trump kissing Mitt's ass."

Read More