The reason it might not be as risky as one might imagine is buried in an otherwise insufferably puffy Grunwald/Caputo piece at Politico:
Rubio and his team do not like to talk about strategy, but in any case, the political calculus of giving up a Senate seat to seek the brass ring was not as painful as it sounds. Even if Rubio doesn’t win the nomination, he could well end up on the Republican ticket. Even if he ends up unemployed in 2017, he can run for governor in 2018 with a Republican-friendly mid-term electorate.
So why not get the requisite training-wheels run for the top spot out of the way? If his campaign never really takes off, it will be attributed to Bush’s strength rather than Rubio’s weakness. And for a dark horse, he’s very well positioned, with surprisingly strong approval/disapproval ratios in the early states—a sign the “base” is ready to accept his backtracking on immigration reform—and the possibility of replacing either Bush—whose own numbers remain questionable—or Scott Walker—one big gaffe or indictment away from Palookaville—in the first tier of candidates.
On top of all that, he’s the candidate Republican Establishment elites are almost certain to drool over if Jebbie blows up or fades. He’s the symbol of change in the GOP, without really making many concessions that strain conservative orthodoxy. A relatively young guy with a Latino background who is (it appears) the closest thing to a Reformicon champion (though again, what Reform Conservatives offer is more an add-on to conservative fiscal policy making it even more fiscally irresponsible than any sea change), and also a favorite of Neocons, is going to get massive positive media attention if and when he becomes more viable.
In other words, should Jeb Bush crash and burn, Rubio is the establishment GOP's plan B. That's not really a great fallback plan given Rubio's dismal polling numbers, but worst-case scenario Rubio succeeds Batboy as governor in the next midterm election, he figures.
Could be worse for him. Marco will land on his feet after he lands on his face.