Politics is a dirty business. Everyone will cut the other guys throat metaphorically whenever the opportunity presents itself. Christie would have been a better president than Rubio in my opinion. But getting to, and being President is a enormously complicated business with more pitfalls than you can count. So far Trump and Cruz are really the only ones who have proved they have the stuff. There is only room for one or maybe two more at this stage.
The Dems are the ones that have no hope. That party destroyed itself.
The dems command support of 48% of this country's electorate and walk into election day with a guarantee of 240 EVs. This country is rife with low-info idiots who, like the semi-trained chimps they are, know just enough to vote for whatever name sports a "(D)." Never, ever count the dems out.