Rubio vs. Trump: It's on, apparently. (AP Photo/John Minchillo)

Marco Rubio issued his most personal criticism yet of Republican presidential rival Donald Trump on Thursday, calling him "touchy and insecure" during a Thursday radio interview that came after Trump ramped up his attacks against the Florida senator.

"He had a really bad debate performance last week," said Rubio of Trump in an interview on Kentucky Sports Radio. "He's not well-informed on the issues. He really never talks about issues and can't have more than a 10-second sound bite on any key issue. And so I think he's kind of been exposed a little bit over the last seven days. And he's a very touchy and insecure guy. And so that's how he reacts. And people can see through it."

While Rubio has occasionally sought to contrast himself with Trump on issues, he hasn't engaged the front-runner as aggressively as some of the other candidates, such as former Florida governor Jeb Bush. Rubio's remarks are particularly notable since he has sought to run a below-the-radar campaign designed to peak early next year just before voting begins, not now.

Trump has intensified his attacks against Rubio this week as The Florida senator's poll numbers have climbed in the wake of last week's debate. Campaigning in South Carolina on Wednesday, Trump said Rubio is "overly ambitious, too young, and I have better hair than he does, right?" He also said Rubio has "no money, zero” and questioned why he is running against Bush, his mentor.

Rubio said Trump "takes shots at everybody that gets anywhere close to him in terms of a poll."