His pursuit of amnesty with the “gang of 8.” Down to playing the race card on those who didn’t like what he was doing.
I'm not a Rubio fan, but this is as good as any example of a problem I see more and more of here on FR: Hyperbole for no purpose.
Rubio joined the Senate in 2011. Comprehensive Immigration wasn't debated until 2013. So this was hardly "the first chance he got."
Comprehensive Immigration failed. So Rubio did not stab his country in the back, even if (as you suggest) he intended to.