Rural areas tend to vote Republican and urban areas tend to vote Democrat. I've often thought that this is for a simple reason. If you live two miles from your nearest neighbor, you're rightfully offended at the thought of your neighbor dictating how loudly you play your music, whether you smoke, and even if you wear pants. By contrast, if you live in an apartment complex with a dozen neighbors within yards of your place in every direction, they have every right to think that they can dictate how loudly you play your music, etc. Population density is a big determinant of what you think is reasonable when it comes to rules and regulations.
This graph seems to support that idea, making increased rates of urbanization one more trend that seems to be working against the GOP.