I really don't believe that fewer people turned out to vote for Romney than they did for McCain. That is as believable as the ridiculous unemployment numbers, and we see that they saw they could get away with that. Remember when they all squealed in unison when someone reputable questioned those numbers?
2.5 million people die every year in the US, and many of them are old white people who vote Republican.
They are replaced (and then some) by non-white people who are voting >70% Democrat.
I'm not totally confident this is the reason, but it is certainly possible.