This photo is from 2012, when Donald Trump thought better of Mitt Romney and endorsed him. AP Photo/Gerald Herbert Former Republican presidential nominee Mitt Romney drew an angry response Sunday from current GOP front-runner Donald Trump.

In an interview on CNN on Sunday, the former Republican presidential candidate said he will back the eventual Republican nominee for president. But he seemingly refused to entertain the idea that Trump could be the nominee.

"Well, I don't think it's likely that Donald Trump will be the nominee, but it's obviously too early to tell. At this stage numbers go up and down, candidates have bursts of support," Romney told "State of the Union" host Jake Tapper.

"Donald Trump has had a lot of support for a long time, and so I think a lot of folks think, 'Well, this could [go] a lot further than, than people had expected.'"

Romney's comments Sunday came after he hammered Trump earlier this weak during an appearance on the podcast hosted by former Obama senior adviser David Axelrod.

"I think Donald Trump has said a number of things which are hurtful — and he has said that they were 'childish' in some respects — and I think [they] will be potentially problematic either in the primary or a general election," Romney told Axelrod.

"And they relate to things he's said about women, and things he's said about members of the news media, things he's said about Hispanics," he added. "I think he'll have some challenges if he proceeds to the next stage."

Despite numerous predictions of his demise, Trump still currently leads the presidential Republican field.

As he often does, Trump responded to Romney almost instantly on Twitter, attacking him over several of Romney's high-profile gaffes during the 2012 presidential race.

Though Trump flirted with a presidential bid in the 2012 cycle, he ultimately endorsed Romney. Since announcing his candidacy earlier this summer, however, Trump has soured on Romney, frequently criticizing the former governor on Twitter.

Romney has apparently been put off by Trump's brash attacks on his fellow Republican and controversial rhetoric throughout the 2016 campaign, and he has said that Trump's comments could damage the party's electoral chances with Latino voters in 2016. Romney notably lost by overwhelming margins among Latino and Hispanic voters in 2012.

"He's someone to whom civility means a lot," a close Romney adviser told New York magazine earlier this year. "The whole Trump thing really bothers him."

Romney wasn't the only establishment Republican to go after Trump on Sunday. Former Florida Gov. Jeb Bush (R), who has slumped in recent GOP polls, criticized the real-estate magnate for his positions on foreign policy. He said Trump was treating his presidential run as if he were "still on 'The Apprentice.'"