To grasp how powerfully demographic change is reshaping the political landscape try this thought experiment about the 2008 election.
Start by considering the electorate's six broadest demographic groups -- white voters with at least a four-year college degree; white voters without a college degree; African-Americans; Hispanics; Asians; and other minorities.
Now posit that each of those groups voted for Barack Obama or John McCain in exactly the same proportions as it actually did. Then imagine that each group represented the share of the electorate that it did in 1992. If each of these groups voted as it did in 2008 but constituted the same share of the electorate as in 1992, McCain would have won. Comfortably...
[P]itch the thought experiment forward 12 years. Imagine that the major demographic groups voted as they did in 2008, but cast a share of the vote equal to their expected share of the population in 2020. (For argument's sake, let's divide whites among college and noncollege voters in the same proportions as today.) In that scenario, Obama beats McCain by nearly 14 points -- almost twice as much as in 2008. Demography will indeed be destiny if Republicans can't broaden their reach.
If you've read much of what I've written over the past few months, you'll know I think this is a very persuasive argument, one that predicts the continued decline of the GOP -- unless they figure out how to start appealing to minorities, of course. For more, see this article I wrote back in November about the boon that the Hispanic vote has been to Democrats in states like New Mexico and Colorado, and why it could even turn Texas into a swing state in a few years.