~311,591,917 Population (in 2011)
~237,744,633 Over 18 (in 2011)
61,218,820 (25.75%) voted Obama
58,200,593 (24.48%) voted Romney
118,325,220 (49.77%) voted neither
Romney got fewer votes then McCain in 2008. Obama got fewer votes them himself in 2008.
Since the population increased since 2011, the "neither" category should actually be somewhat larger,
most likely breaking the 50% barrier.
A sizeable group of both romney and obama voters must have also thought they are voting for the "lesser evil"
or "the shinier of two turds". This further increases the amount of the population who hates both parties.