He goofed. You really do need to see the video to appreciate how bloody a takedown this was. It’s the worst debate performance of this election cycle.
Senator Rubio finished third in Iowa and arguably came out of the experience with more momentum than any of the other "establishment" candidates. But he has been criticized for the fact that he sticks a little too tightly to his "message." Every politician has a line or a theme which s/he can pull out in a pinch, but Rubio is a particularly extreme case. Reporters who follow him on the trail say that nearly everything that comes out of his mouth is scripted and prepared. Governor Christie as been trash-talking him all week about this and finally got the chance to knife him.
Rubio's answers in every debate have followed a particular script: he gives a prepared-but-vapid reply and then transitions to one of several (similarly scripted, but slightly longer) speeches designed to (1) sound nationalistic and (2) take a crack at Barack Obama or Hillary Clinton. Both set him up as a plausible general election candidate. If he’s attacked, he swats back with a pre-prepared insult before proceeding to his speech.
This actually isn’t a bad strategy if you’re a candidate who is as uncomfortable debating as Marco Rubio apparently is, but it leaves you unprepared for new situations and attacks. Keep that in mind, while you consider what actually happened last night.
Early in the debate, Sen. Rubio was asked a question about his accomplishments and his experience as a first term Senator. He answered according to his programming as described above. Prepared, vapid reply:
>Well, let me say, from protecting the people of Florida from eminent domain abuse, to bringing accountability to the V.A., to the Girls Count Act, to sanctioning groups, I'm proud of my service in the United States Senate and before that, in the Florida legislature.
Most people won't look very deeply into his list of accomplishments, and Rubio knows it. He just needed something to say so he didn't look unprepared. Then after a brief digression on Joe Biden, pivots to his longer speech. Notice the attack on Obama and the nationalism, in bold. Obama is making America like Europe. Truly, he is history's greatest monster. Red meat for the GOP audience.
>... And let's dispel once and for all with this fiction that Barack Obama doesn't know what he's doing. He knows exactly what he's doing. Barack Obama is undertaking a systematic effort to change this country, to make America more like the rest of the world.
>That's why he passed Obamacare and the stimulus and Dodd-Frank and the deal with Iran. It is a systematic effort to change America. When I'm president of the United States, we are going to re-embrace all the things that made America the greatest nation in the world and we are going to leave our children with what they deserve: the single greatest nation in the history of the world.
His argument is that experience doesn’t matter because Obama managed to push a liberal agenda in spite of his lack of experience and fervent GOP opposition. (See here and here for post-debate elaborations re: Rubio's basic point). But it’s a clumsily stated argument and isn’t a good fit for the question. Obama's inexperience is a GOP meme (even after seven years) but at this point in the evening nobody had mentioned it. He mangled his transition so badly it isn't clear why he even brought it up. (edit: So at this early point in the exchange, Rubio is already on the back foot, even before he starts to loop and well before Christie has started to speak. It's an unforced error.)