In a word, "Ryan." It was obvious this pick made no sense. If you wanted dependable, there was Rob Portman who might bring you Ohio. If you wanted a risk, Marco Rubio was the man, a very good speaker with Hispanic routes who might bring you Florida, even if his past contained some sketchiness.
But Mitt went and chose the risk that brought with it no return, unless you count giving voters the vague impression you want to smother anyone over 65 with a pillow.
Here is Howie Klein's take on the hardening conventional wisdom this Ryan was Romney's biggest disaster (among a list of too many to recall). It is worth clicking on just for the picture accompanying the post:
"Right-wing propaganda expert Byron York rues that Ryan can't even deliver his own states and wonders aloud, in a GOP throw away, whether Portman could have at least brought in Ohio for Romney. "Amid all the talk among conservatives that Romney is not making good use of Ryan-- that Romney's campaign team is muzzling Ryan, keeping him from stressing the budget and entitlement reforms that are his life's work-- listening to Portman on the stump is a reminder that Romney could have chosen a different path. Especially since Portman, whose presence conveys experience and dependability, is a known commodity in a state that is at or near the top of Romney's must-win list. Ryan, whose youth often overshadows other impressions he makes on voters, understandably doesn't have the same status in Ohio as its home-state senator."
A general consensus is beginning to form that as bad as the 47% blunder was for Romney, the Ryan selection was worse. The New Republic's Noam Scheiber writes that as bad as the 47% disaster was for Romney, there's "another dynamic that’s been overlooked here: The escalating disaster that is Paul Ryan. At the time of his selection, a number of pundits argued Ryan’s strategic benefits, suggesting he would boost Romney by energizing conservatives, or by allowing Romney to run as the candidate of big ideas, or that he would at least be the party’s best defender of the Medicare plan Romney was going to have to defend whether he wanted to or not. This seemed like a stretch at the time-- after all, Ryan’s Medicare plan proved to be a massive liability the one time voters weighed in on it."