It could easily have been something like a breath mint. I would imagine it gets very hot under those lights, and it is difficult to speak when your mouth gets dry. If not that, it was probably a beta blocker. Many speakers and musicians use them to combat shaking and breathlessness when under stress. Beta blockers don’t cause any unwanted effects, like hyperactivity, drowsiness, or impaired cognition, and they are very effective at removing visible signs of nervousness.
....”Beta blockers for breathlessness when under stress”.....
That’s interesting....I noted when Rubio does interviews he takes breaths often and notable as he speaks.