I think there is a case for Rubio and Carson doing this, since Cruz is probably acceptable to their supporters. It would probably be best for the others to stay in though, as their supporters would probably just not vote at all if they exited, and that could allow Trump to cross 50% in some of the states that have a winner-take-all threshold.
Of course, that is assuming those candidates would prefer Cruz over Trump, which is far from certain.
Rubio is a senator, and, thus, it is very likely he would prefer anybody other than Cruz to win. That anybody would include Donald Trump, Vladimir Putin and Devil Reincarnate.
I don't necessarily agree with that. I know Cruz has earned his reputation as probably the most despised member of the Senate, but it isn't as though Rubio hasn't stepped on plenty of toes as well. Rubio and Cruz both came to the Senate as Tea Party-backed insurgent candidates, and they are very close on a number of policy issues. If Rubio feels that the establishment is starting to cozy up to Trump, I could see him dropping out and endorsing Cruz as a way of metaphorically giving the establishment the finger for abandoning him.
The two of them have had some pretty bad spats over immigration and national security in the debates.
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