That's not a problem in itself. To the contrary, the fact evangelicals and wealthy voters are so overwhelmingly Republican means that they have this electorate locked up and can focus on convincing more "swingy" demographics.
Doesn't means they will (at least not anytime soon).
Well, there's two sides to that coin. If the GOP primary electorate were even slightly more rural this year, Santorum would have been the nominee. The party is getting more rural with time. Obama more than compensated with his margins in the suburbs, but one of the big stories of 2012 was the complete collapse of rural white Democrats outside of New England. Just a few more rural voters in the GOP primary and the SoCons may have a stranglehold by 2016.
It's similar to the predicament of the early 20th century Democrats. Their rural support was so strong that they would nominate candidates completely unacceptable to the cities and suburbs. I don't expect the modern GOP to go that far off the cliff, but this is fundamentally the reason they can't break 48% right now.
The best test of how bad it will get is the South. If the South gets even more one-sided to the point where the less diverse states are consistently going 70% R, then it will take a long while to get out of the SoCon trap. As I see it there are two ways back to the 50 state campaigns of the mid-20th century:
1. The rise of a religious left, which allows Democrats to seriously contest the Evangelical vote, forcing the GOP to expand its coalition and making the Deep South competitive again. A Great Awakening style revival historically follows the kind of economic turmoil the country has recently faced, and the movement would probably have a progressive bent given today's youth.
2. The parties make peace on environmental issues or new technologies solve pollution/climate change to the point where green issues fade from the political scene. In this world, Republicans could contest the West Coast and Democrats could compete again in Appalachia.
