Rubio's vulnerability is so great in part because he staked so much on immigration reform as a way to loft himself to the top tier of 2016 GOP candidates. But the other part is because there was so little to the man in the first place absent his fortuitous would-be positioning as the young new Hispanic face of a Republican party reeling from a reputation for having little to no traction with America's burgeoning non-white population.
Remember, Rubio was basically an accidental Senator, swept up in the floodtide of the 2010 Tea Party mid-term, though it's true that many careers start that way.
Now that it's clear that the base of the GOP - as expressed in the House GOP's diehard desire to kill reform - is emphatically not on board with the Senate immigration bill to which he tied his fate, his whole plan for the 2016 run is basically in a shambles and his support among conservatives is falling rapidly. (His national numbers, rather than Republicans only, show similar problems: see chart.) There's even a hopeless and unconstitutional movement afoot among Florida conservatives to 'recall' him from office. (Sorry, you can't recall federal officeholders. Been there, done that.)
So what to do?
If you've watched over recent weeks, Rubio has been casting around for basically any right wing position to grab on to. First, there was his quixotic demand to shut down the government if Obamacare isn't defunded. Then yesterday he tried to filibuster the Perez nomination even after the Senate nuclear option deal. And there have been various other examples. But as Dave Weigel aptly notes, now every desperate lurch right looks self-serving, cynical and well ... desperate.
So now Rubio seems trapped, on the wrong side of his party's base on a key issue - and one that looks unlikely even to deliver legislation that might have bipartisanship traction with middle-ground voters. It's one thing to say 'I bucked my party to bring change the country needs', another to say 'I bucked my party on change my country needs but it actually didn't pan out. Sorry.' And now he's forced to become some sort of hyperactive conservative wild man - what he wasn't supposed to be - in order to recoup ground on the right that likely can't be salvaged.