Donald Trump has a new, surging rival in his crosshairs

Real-estate developer Donald Trump is now frequently attacking a presidential rival he mostly refrained from criticizing in the past: Sen. Marco Rubio (R-Florida).

During a Thursday morning CNN interview, Trump hit Rubio as a "kid" in a wide-ranging critique.

He first bashed Rubio's credibility as a foreign-policy wonk when asked to compare their knowledge about Syria.

"Marco Rubio sits behind a desk sometimes and he reads stuff," Trump said, according to CNN's transcript. "That's all he does. I create jobs all day long. I'll know more about all of this than all of them put together and believe me, we'll have a winning strategy. If Marco Rubio is good, how come we're doing so badly?"

Trump went on to dismiss the 44-year-old senator's age.

"These guys don't know how to win," he said. "Marco Rubio, he's like a kid. He shouldn't even be running in this race, as far as I'm concerned. He's a kid."

Later in the interview, Trump again called Rubio a "kid" and criticized him for not deferring his candidacy to Jeb Bush, the former Florida governor.

"He's a kid. He has no right - I mean, frankly, he was very disloyal to Bush," Trump said. "Bush was his mentor. And everyone said he'd never run because Bush was his mentor. Well, the fact that Bush was his mentor didn't stop this young guy who is overly ambitious from wanting to run. Now Bush looks foolish and he looks like he's a very disloyal guy, frankly."

Earlier in the day during a "Morning Joe" interview, Trump also mocked Rubio for perspiring during last week's presidential debate.

"I'm looking at guys like Marco Rubio, who has the worst voting record in the United States Senate. And [he's a] young guy - although he sweats more than any young person I've ever seen in my life," Trump said. "I've never seen a guy down water like he downs water. They bring it in in buckets for this guy."

Rubio has surged in national polls since a strong performance in last week's Republican presidential debate, and he moved up to second place behind Trump in his home state of Florida. He and Bush are the establishment favorites in a GOP field still dominated by the real-estate magnate and, to a lesser extent, fellow political outsiders in retired neurosurgeon Ben Carson and former Hewlett-Packard CEO Carly Fiorina.

Trump frequently brags about being a "counter-puncher" during speeches and interviews, meaning he tries to wait until he is criticized before hitting back hard. This has prompted Trump to trash almost all his rivals at various points in the campaign, with a couple of exceptions.

In Rubio's case, the senator recently said Trump had yet to "talk seriously about national security." Trump responded by unloading on Rubio earlier this week: