Rubio already alienated the Republican party base. The GOP-e may think he's a top candidate, but hard as it might be for some to believe, the base doesn't like Romney, and they now dislike Rubio.
That's hardly a formula for victory.
.....the base doesn’t like Romney, and they now dislike Rubio.
And the base dislikes 0bama more than either.
The base will vote for Romney and whoever.