Eliana posted yesterday on the Rubio SuperPAC hitting Christie. This goes to a dilemma that Rubio has: he’s in competition with almost every other major candidate, which is a downside to trying to straddle the divide between the grass-roots Right and the establishment. Rubio is competing with Cruz for very conservative voters, at the same time he is competing with Christie, Bush, and Kasich for center-right voters, and on top of this Trump and Paul fervently hate him on immigration and national security respectively. This means that, like Romney in 2008, Rubio is the candidate that everyone can agree to attack (in 2008 this reflected a personal dislike the other candidates had for Romney).
We got a taste of this dynamic in the last debate, when Rubio, running thirdish in the race, took incoming as if he were the frontrunner. Rubio can certainly run this gauntlet, but it’s much easier to be Ted Cruz, especially so long as Donald Trump is relatively quiet.