EMERGING DEMOCRATIC MAJORITY…. George W. Bush, among others, has spent a fair amount of time of late urging the Republican Party to be more open to diversity. There’s a very good reason for that: based on Census Bureau data, the white share of the population will drop to 60% by 2020, and 51% by 2040.

With that in mind, National Journal‘s Ron Brownstein has a fascinating item this week about demographic trends in the United States, and their electoral implications. (via Koppelman)

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.

That’s because Obama’s best groups are much larger today than in 1992…. The best way to illustrate that prospect is to pitch 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.

The most reliable Republican voters in the country are blue-collar whites without college degrees. It’s the group that’s also shrunk the most between 1992 and 2008 — from 53% of the population to 39%.

The New York Times noted the other day that the Republican Party hopes to “avoid shrinking into a party of Southern white men in an increasingly diverse country.” Given the demographic changes, it sounds like the GOP should probably do more than just hope.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.