But, over time, you'll start to notice that the paragraphs that Rubio speaks in start to sound a lot alike. That's because they are. And, you'll start to see Rubio as less the smartest kid in the class and more as the kid who memorized every answer in the book but doesn't have much of a clue about what it all really means.

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That latter version of Rubio was the one on display in Saturday's ABC debate.

He went from cool new song you can't wait to hear again to broken record playing the same song for hours that makes you want to jump out a window. That Chris Christie called Rubio out so directly on the "Barack Obama knows exactly what he is doing" line made it all the worse for the Florida senator.

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Rubio sought to defend himself on the Sunday talk show circuit, refusing to apologize for his robotic appearance on Saturday night. "It’s what I believe," he told ABC's George Stephanopoulos on "This Week." "And it’s what I’m going to continue to say because it happens to be one of the main reasons why I am running."

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The danger for Rubio in this race is -- and always has been -- the implicit and, at times, explicit, comparison between himself and President Obama. Jeb Bush's campaign has been calling Rubio the Republican Obama for quite a while now; the Florida senator has made clear he knows it's not meant as a compliment "and I certainly wouldn't take it as that."

In the eyes of most Republicans, Obama talked a great game but showed his inexperience and, they would argue, arrogance once in office. They say he was ill-prepared for the job, but people didn't know that because he was such a gifted communicator. A mile wide and an inch deep and all that.

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Rubio, knowing that his age (at 44, he's the youngest candidate in the field) and his charisma were always going to evoke the Obama comparison, has spent much of the race doing everything he could to cast himself as a serious policy thinker -- someone who, while young, was deeply engrossed in the complexities and challenges of not only the United States but also the broader world.

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His critics -- mainly his rivals -- insisted that all of Rubio's smart talk was the thinnest of veneers, that when pressed the guy would show that he lacked the depth to be commander in chief. I'm not sure Rubio entirely proved his critics right on Saturday night. One debate does not a campaign end.