PORTSMOUTH, N.H. — The longstanding feud between real estate mogul Donald Trump and former Florida governor Jeb Bush has escalated to a particularly nasty level on the eve of the Republican presidential primary here in New Hampshire, as the two publicly traded insults Monday that were decidedly more personal than political.

“Jeb is having some kind of a breakdown I think. And, look, he's an embarrassment to his family,” Trump told CNN’s Wolf Blitzer Monday afternoon. “He has to bring his mother out and lug his mother around at 90-years-old. I think it's a very sad situation that's taking place.”

The billionaire showman regularly belittles Bush on the stump, a peculiar target given the former governor’s low standing in national polls. Bush was once the presumptive Republican front-runner in this election, but his support withered as Trump captured the attention of anti-establishment conservatives across the country.

Bush fired off an aggressive tweet at Trump Monday, telling him “you aren’t just a loser, you are a liar and a whiner." He followed up that criticism during an interview on CNN Monday afternoon in which he defended his tone and added that he is the only candidate who has repeatedly called out Trump for what his detractors say is highly divisive rhetoric.

“Well, he is a whiner, and I'm defending the honor of people that I really respect. And I think that's more than appropriate,” Bush said on CNN Monday.

Trump, who gave a live telephone interview immediately after Bush’s interview with the network aired, said that Bush was a “failed candidate” and a “laughingstock."

“He's a desperate person. He's a sad and, you know, he's a pathetic person. He doesn't even use his last name in his ads. He's a sad person who has gone absolutely crazy,” Trump said. “I mean, this guy is a nervous wreck. I've never seen anything like it.”

“He says nasty things. I always put him down on the dais and he goes away like a little sheep,” Trump added later.

The New Hampshire campaign is a chance for Bush to rehabilitate his badly bruised candidacy; a victory here could theoretically clear that crowded field of establishment candidates in the race and give him momentum before nominating contests in South Carolina and Nevada. But polls paint a grim picture for the Bush campaign — the former governor trails behind Sen. Marco Rubio (Fla.), Sen. Ted Cruz (Tex.) and Ohio Gov. John Kasich, and many voters on the ground have expressed an intense unwillingness to vote for another member of the Bush family.

Bush landed several blows on Trump during Saturday night’s GOP presidential debate, knocking him for his position on eminent domain, an admittedly wonky subject but one on which Trump has been politically vulnerable. Though Trump has provided full-throated support of the important public role eminent domain serves, including building highways, he stumbled Saturday when pressed by Bush on his attempt to use it to build a casino parking lot in Atlantic City, N.J., by taking property from an elderly woman.

Trump fired back Monday, saying that Bush's family has benefited from eminent domain through their share in the Texas Rangers Stadium.

“He didn't use it because he's not smart enough to use it ... but how can he criticize me for using eminent domain and his own family used it?” Trump said.