Republican Florida Sen. Marco Rubio on Sunday refused to explain how a potential Romney administration would be different from George W. Bush's administration, but insisted that the last Republican president "did a fantastic job" and his brother, former Gov. Jeb Bush (R-FL), would be a "fantastic vice president."

"I haven't gone through the comparison," Rubio told CNN's Candy Crowley. "I think that presidents serve in different times with different challenges. And so I think that George W. Bush, in my opinion, did a fantastic job as president over eight years, facing a set of circumstances during those eight years that are different from the circumstances that a President Romney would face."

Last week, Jeb Bush told Newsmax that he would "consider" a vice presidential nomination, but suggested Rubio was "probably the best" choice.

"Well, that's very nice of Jeb," Rubio said on Sunday. "I hope he'll say yes if future President Romney asks him," adding that the former Florida governor would be a "fantastic vice president."