To start, most Republican voters are quite old. The party will probably lose over a third of its members to old age in the next 20 years and has only tiny base of younger voters to replace them.
Age Range Percent
The baby boom has moved through the population pyramid, and these older, more conservative voters are decreasing in number as age takes it toll.
Next, their voters are also very, very white (90%!!):
This has been trending against them for at least the last 3 elections. The exit poll data from CNN for Presidential elections by Race:
Election Year Percent White
Finally, this trend is looking to continue well into the future
What does this all mean? Every year it will be harder for GOP to win elections by appealing to the racism, intolerance and religiosity of the electorate. The re-election of Barack Obama is a refreshing reminder that the old guard is still indeed very powerful, but the new generation has found its voice.