It seems the urban areas favor Blue and rural and fringe suburbs swing Red.
80% of Americans live on the wetter side east of the 100th meridian which runs roughly from Winnipeg down through El Paso. 
That leaves 20% on the dry west side. duh
So that's 250 million east and 80 million west. But there is 50 million hugging the moist west coast.
That means 30 million in the dry zone which encompasses 8 states and pieces of 9 others.
That's 16+ senators for less than 10% of the population. Hmmm.
However 10 million out of that 30 live in Phoenix, Denver, and Vegas.
Another 5 million live in Salt Lake, Tucson, Albuquerque, El Paso, and Boise so that's half the 30 million where there's a chance of some Blue there. (burbs count). 
Remember nothing is written in stone, I'm talking trends which is on par with reading tea leaves.