Real-estate tycoon Donald Trump and former Florida Gov. Jeb Bush (R) exchanged jabs in separate "Morning Joe" interviews Monday.
Bush, who has been Trump's most consistent campaign-trail critic, blasted some of his GOP competitors for not similarly going after Trump.
"It's ridiculous," Bush said after suggesting his opponents were scared of Trump.
"The guy's just a real-estate guy," Bush continued. "He's gone bankrupt four times. He's a successful man but he's not perfect by any stretch of the imagination. So why not confront him and challenge him?"
The MSNBC show then played a word-association game with Bush. The first word: "Trump."
"Loser," Bush replied without hesitating.
Trump, the Republican presidential front-runner, called into "Morning Joe" for a subsequent interview. Trump fired back at Bush, whom he repeatedly called a "stiff."
WATCH: Trump responds to Bush 'loser' label:
"Jeb is an interesting duck," Trump mused. "Here's the story on Jeb: He's a stiff who you wouldn't hire in private enterprise, OK? This is a stiff. This is a guy that if he came looking for a job, you'd say: 'No thank you.' And that's the way it is."
After accusing Bush of lying about his record, Trump added:
%shareLinks-quote="The guy's a total stiff. He's not going anywhere. Remember this -- for the voters -- he wants Common Core and he's very weak on immigration. Remember, ... he said, 'They come to our country through the act of love.' Jeb is weak on immigration." type="quote" author="Donald Trump" authordesc="" isquoteoftheday="false"%
Both Bush and Trump have been focused on Tuesday's primary in New Hampshire.
See more from Saturday's GOP debate: