FRANKFORT, Ky. (AP) — Republican presidential candidate Marco Rubio jabbed billionaire businessman Donald Trump as "touchy and insecure" on Thursday, joining the chorus of Trump critics who have intensified pressure on the GOP front-runner in recent weeks.

The Florida senator also challenged Trump's command of the issues during an interview with Kentucky Sports Radio, the most widely listened-to sports talk show in the state. Before Thursday, Rubio was among the few Republican White House hopefuls who largely avoided engaging with Trump.

A day earlier, Trump had called Rubio a "lightweight" while campaigning in South Carolina and criticized his lack of foreign policy experience. The former reality television star has lashed out at several Republican rivals in a campaign consumed by personal attacks at times.

Breaking his silence on Trump, Rubio responded Thursday that the real estate mogul relies on attacking people.

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

Appearing later on the Fox News program "The O'Reilly Factor" Rubio repeated his jabs against Trump when pressed by host Bill O'Reilly if the GOP front-runner has enough "depth" to deal with major issues facing the country.

"He can't have a conversation about policy because, quite frankly, he doesn't know anything about policy," said Rubio, who asserted that Trump's "foreign policy apparently is a secret he can't tell us because he doesn't want to reveal it to the enemy."

Rubio is one of several Republican presidential candidates taking an interest in Kentucky since the state party voted to have a presidential caucus on March 5 instead of its traditional primary in May.

The shift allows Republican contender Rand Paul, the state's junior U.S. senator, to work around a state law banning candidates from appearing on the ballot twice in the same election. Paul, who is up for re-election in November, paid the state GOP $250,000 to cover the cost of the shift.

The caucus has generated interest from other candidates given Paul's continued slide in the polls and the caucus' unusual rules that allow candidates to claim delegates with as little as 5 percent of support. Thursday, Republican candidate Jeb Bush visited Louisville for a state party fundraiser and became the first candidate to pay the $15,000 filing fee to participate in the caucus.