I think the world of Senator Marco Rubio. In personal style and political positions, he ranks among my favorite senators. I supported his insurgent campaign personally and financially from its earliest days and think he's the future of the conservative movement. When and if he forms a 2016 or 2020 exploratory committee -- as I expect he will -- I'll almost certainly donate to it. That said, I think that it would be a mistake for Mitt Romney to pick him as his running mate in this election cycle. There are three reasons for this: first, the fit is bad, second, it's won't help Romney win; finally, Rubio isn't ready to be president.
Romney is a technocrat at heart. This, in my mind, is mostly a good thing. But Rubio isn't one. As I've written out before, however, Romney would do best to pick a "mini-me" who thinks and act like him. But the exciting, inspiring Rubio doesn't seem like he would be a good partner for the dry-as-dust Romney. It's possible the two would strike up an unlikely friendship, such things have happened, but Vice Presidential candidates who don't get along with the person at the top of the ticket -- Sarah Palin, John Edwards, and Joe Lieberman -- often prove a distraction for the person at the topic of the ticket.
Just as importantly, I doubt that a Rubio nomination will make much of a difference in the election. While it's possible that Rubio could deliver Florida for a Romney ticket, the state doesn't actually seem to be key to the election. Looking at the maps, it appears that Romney will have to take all current "swing states" -- including Virginia where Obama currently, and somewhat surprisingly, has a decent-sized lead -- plus either Ohio or Michigan. If Romney can't take at least one of these two rust-belt states, it's difficult to see a path to victory for him. On the other hand, if Romney can't win either Ohio or Michigan (or, say, some other not-currently-in-play state like Pennsylvania), Florida won't matter. In fact, given the expense of running a campaign in Florida (the state has five big media markets), it could even, strangely, benefit an Obama campaign if it pulls out of the state and devotes more resources to the rust belt states that are more likely to decide the election.
Finally, it's difficult to see how a Rubio nomination would help the Senator himself. And, as one of the GOP's brightest stars, his own future should matter to all conservatives. Rubio, with only two years on the national political stage, isn't really qualified to take over the White House. (Neither was Obama.) He needs more seasoning in the Senate and it would do well if he could develop a meaningful legislative record and foreign policy experience. (The former is nearly impossible in the current hyper-partisan, Democrat-dominated Senate.) And, despite having some clear, well-articulated foreign policy views, Rubio (correctly focused on being a Florida Senator rather than his own national ambitions) hasn't done nearly the amount of foreign policy groundwork that Obama did before his presidential work. This is a huge credit to Rubio himself but does raise doubts as to whether or not he's really ready to be Preisdent.
Marco Rubio will only be 45 on Election Day 2016 and 49 on Election Day 2020. He has plenty of time to run for the Presidency and I'd put even money on the notion that he'll be the first Latino occupant of the White House. But both he and the Republican Party would be better off picking someone else for the number two spot on the 2020 ticket.