Story highlights Donald Trump and Jeb Bush have been feuding since the billionaire businessman entered the race

Trump calls Bush a loser, just a day after Bush called Trump a loser

(CNN) Donald Trump returned to mocking Jeb Bush after the two clashed during ABC's Republican presidential debate on Saturday night, calling him a "loser" and saying "every time he attacks me he melts like butter."

Appearing on "CBS This Morning" on Monday, Trump was dismissive of the contentious exchanges with Bush during the debate, and of Bush himself.

"You look at Jeb -- Jeb's a loser," Trump said. "He's spent $110 million so far, he's at the bottom of the pack or toward the bottom of the pack. He's going nowhere. And the only thing he does is attack me."

Trump and Bush have had a running war of words for most of the time Trump has been running for president. The two sharply attacked each other during Saturday's debate, and Bush called Trump a loser on the campaign trail Sunday.

Trump responded Monday by calling Bush sad.

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