Republican presidential candidate Jeb Bush said Sunday that GOP front-runner Donald TrumpDonald John TrumpMinneapolis erupts for third night, as protests spread, Trump vows retaliation Stocks open mixed ahead of Trump briefing on China The island that can save America MORE is not serious enough to be president.

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"I have great doubts, to be honest with you," Bush said during an interview on CNN's "State of the Union" show.

"It looks as though he's not taking the possibility of being the president of the United States really seriously," Bush continued. "For him, it looks as those he's an actor playing the role of a candidate for president, not boning up on the issues, not having a broad sense of the responsibilities of what it is to be a president."

Bush has lagged behind Trump and other outsider candidates such as Ben Carson and Carly Fiorina in polls for months, but he said did not think Trump would be able to go the distance in the GOP presidential nomination fight.

"Look, he's a phenomenal personality for sure, and he's capturing people's deep anger and angst about Washington, D.C., for sure, but he's not going to be able to solve these problems," Bush said. "He's mirroring people's anger, and he does it very effectively.

"I don't think Trump's going to win the nomination," he continued. "I think we're going to have a nominee that will unite the party and win the presidency, but when people begin to think about who is going to be the next president, has the judgment and the experience to lead us in a different direction, I think his support is going to wane."

Bush also weighed in on the first Democratic debate, saying Democratic front-runner Hillary Clinton looked "beatable" despite her widely-praised debate performance.

"She looked beatable because of the ideas she's embraced," Bush said of Clinton. "She did a good job in the debate for sure. She's smart person, no doubt about that. But every time she had a chance to lay out a different approach than the one we're on now, more taxes, more regulation, more creating barriers on people's right to rise up.

"If I'm elected the Republican nominee, it will be a stark contrast in, do you believe the country's future is a hopeful, optimistic future that gives people the opportunity to achieve earned success, or should we manage the decline?" Bush concluded.