The theory goes like this: GOP turnout was actually fine, up from 2008 (which was indicated by the early exit polling, though to a lesser degree than I would have expected, but exit polls tend to overweight Democrats anyway). My hunch is that independents failed to show up.
See, you get them on the phone, and you ask them questions, and they say they prefer Romney to Obama. But maybe not by much -- not enough to get out and vote. So while both parties turn out their base, the "mushy middle" is just mush, and decides in the end that they don't feel strongly enough to vote after all.
That would explain a large part of the over 10 million vote drop overall from 2008, and Romney's lower share of the vote than projected, because his lead among independents wound up not mattering if independents failed to turn out significantly enough compared to the party bases.
Again, this is strictly a theory off the top of my head, with no data behind it yet. But if true, it shows the folly of chasing the mushy middle.
God you would think that the last four years would have spurred people to get off of their fat duffs and voted... This is going to be a disaster in the making!!!