I'd say that the incident where he repeated himself during the NH debate ("let's dispel with this fiction that Obama doesn't know what he's doing. He knows exactly what he's doing") really damaged him and made him look like an empty suit
He was doing that the entire campaign trail, Christie observed him carefully and just called him out on it, rubio thought he was doing the right thing repeating himself at the time because he had been prepared and rehearsed to do a select response to an expected question.
https://www.youtube.com/watch?v=yrID5W0y3kwhttps://www.youtube.com/watch?v=sZVbhWty_X8You can see Christie shows some begrudging respect for Cruz. But Rubio didn't just repeat it once, he did it multiple times, not only in the debate, but also in media appearances the next day as well.
This was long documented in the media:
http://www.vox.com/2015/12/23/10658566/marco-rubio-new-hampshireBasically, the guy was so used to do doing this on the campaign trail that he was shook up when finally attacked on it and questioned aggressively.
Of course, if rubio had been less scripted and rehearsed in the first place, would he have done is well? Debatable. It's a double-edged sword after all, you get media soundbites with one method, but not with the other.
The point is that it was an inherent pattern characteristic to his campaigning strategy, it wasn't some impromptu remark.
