Washington (CNN) Sen. Marco Rubio kept up the heat on Sen. Ted Cruz Wednesday morning, blasting the Texas Republican for "talking tough" but not being able to follow through on his promises to beat ISIS.

"On the national defense of this country, he talks very tough about how we're going to make sand glow in the desert. The problem is you can only do that if you have an Air Force and we're gonna be left with the oldest, smallest Air Force we've ever had. And that would only be accelerated by a budget he supports," Rubio said on Fox News.

The fight between Rubio and Cruz took center stage at the fifth Republican debate, elevating a long-anticipated battle in the Republican field. Rubio knocked Cruz Tuesday night, accusing him of voting in favor a law, the USA Freedom Act, which the Florida Republican said deprived intelligence agencies of tools for tracking terrorists. But Cruz fired back on stage, saying that the law actually "strengthened" the U.S. government's hand.

It was their exchange on immigration -- with Cruz blasting Rubio for supporting the "Gang of Eight" immigration reform bill, and Rubio hitting back alleging Cruz supports amnesty for undocumented immigrants -- which drew the most attention.

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