Rubio has the benefits of speaking well and clearly and being a very attractive candidate on TV (which gets him heard by more people than would bother paying attention otherwise). When he speaks, he comes across as intelligent and knowledgable. Another important factor in actually being heard. These qualities, I believe, will stand up regardless of this kind of inside baseball - most voters will not care about him not running for the Senate in Florida. And if he is the standard bearer, I don't think Florida voters will hold that against him in the Presidential vote...other things they might (his position on a couple of hot issues there) but not this I think. Bigger things will overwhelm this...what do we call it, a procedural issue. There are plenty of good reasons why he wouldn't run for two offices at once.