They got the president. If Obama gets fully involved in the campaign - I think he will probably turn the tide himself. The message of hope is starting to re-resonate with a lot of people (kicked off by the hyper smart SotU this year) - and if he gets out fully on the trail - I don't think even a fully ready Rubio + RNC can stop him. But I don't know for sure how hardcore he will be campaigning. (He might have to, you know, do his job or something)
There's a reason Castro is being talked up as a potential VP pick (this is why I think going with Warren would be disastrous for Clinton)
But the GOP is gonna go "Hey, are any of those people in leadership? Got any governors? Anyone who has responsibility?" The part people aren't fully getting I think is this: you don't need to get the majority of latinos or blacks or asians to vote you. You're talking 10% (on an absolute scale, not relative) more on each one of those, and, bam, GOP win. If black voter turnout drops to non Obama levels? Even less.
I don't think Cruz can get that; I don't think any of the candidates besides Rubio (establishment backing, good looking, young, etc) and Trump (because I am never counting him out after watching the last few months) have a chance against Clinton. But people are really, really counting the GOP out, even though, by all historic measures, we're primed for a GOP president.