The most compelling demographic analysis I've seen lately: Alan Abramowitz's "The Incredible Shrinking Republican Base" on Real Clear Politics.
Short story: White Married Christians–the combined demographic–is and has been the base of Republicans' victories. Ignore income. Ignore gender. Ignore age. If you're white, married, and Christian, the odds are six or seven in ten that you'll vote Republican.
And that demographic is in precipitous decline. It's below 50% for the first time in (modern?) history.
I saw another telling statistic the other day: for the first time ever, among children in the US under five years old, less than 50% are white.