Jeb Bush: 'It is not comfortable for me to attack Marco Rubio'

(CNN)Jeb Bush alternated Monday morning between defending his GOP primary rival Marco Rubio from criticism of his debate performance, and also criticizing his one-time protege for being untested.

"In politics, when there is a validation of what the perception is, it is bad news," Bush told CNN's "New Day" on Monday. "In Marco's case, look, he is not as scripted as he came off in that debate. He is a gifted speaker. He is a gifted person."
Bush added, "but, he has nothing in his background to suggest that he could make a tough decision."
Bush said that the Florida senator's struggle was proof that the race for the Republican nomination remains open, and that it's "important that all of us get challenged."
    CNN's Chris Cuomo, nodding to Bush's personal history with Rubio, asked Bush how he felt watching Chris Christie blast Rubio over his remarks during the debate.
    "He is a friend," Bush explained. "It is not comfortable for me to attack Marco Rubio. But I do think that everybody needs to be challenged because I'll tell you what, whoever is the Republican nominee, they better fasten their seat belts. Because the Clinton hit machine is coming straight at you."
    Bush also pushed back on the notion that Rubio had seized the establishment mantle in the nomination race after his stronger-than-expected third place finish in the Iowa caucuses.
    "There's no coronation here. He is not going to get a layup," Bush argued. "The whole Washington establishment can't just say part the waters, Marco's coming."
    "The debate on Saturday proved why that is important that all of us get challenged. We have a long way to go."