Rubio does well in debates, and his put-down of Jeb Bush - "someone convinced you attacking me is going to help you" - was further evidence that, unlike the senior politico, he can actually do politics and is an articulate candidate to boot. Yes, the Republican race at the moment remains dominated by the extraordinary - as in extraordinarily ridiculous - Donald Trump, with retired neuro-surgeon Ben Carson in second place. But there still exists a sense that when the votes start being cast, Trump might flounder, while Carson's lack of political experience is already causing him problems. Then the Republicans will start looking for their serious, Hilalry-beating candidate. And at the moment that is looking like the senator from Florida.
Rubio is young, fresh, runs to the Republican conservative base without sounding or appearing overly wacky, and as the son of Cuban immigrants has a Latin appeal that is electorally powerful.
The Bushes deride him as a "GOP Obama", but that might seem to those outside the Republican bubble as a positively good thing. Just as the fresh young senator from Illinois dispatched a member of his party's political aristocracy, it looks as if Rubio might just do the same. If he does, Hillary really should be worried. After all, her track record against dynamic incomers is hardly encouraging.