Rubio is the latin version of Sarah Palin, no experience and a light weight. Plus, he promises to deliver something that no one can promise to deliver, Palin the female vote, Rubio the Florida/Latin vote.
Polling suggests Romney is closing the Mexican/Latin gap with Obama, so Rubio may be useless. I think he will pick a Portman or some other solid governor who has great experience, is middle of the road and will look presidential next to Romney and make the ticket very strong. You will not see the same mistake that McCain made.
If Europe continues to fold in on itself, which is completely out of Obamas hands and our economy falters because of this, and it will, Romney will win going away.
As it stands, it is too close to call in most polls right now and Romney is not a very strong candidate. Imagine if Romney could give a speech like Obama, he'd be ahead by 25 points and Obama would just quit.