Internal migration among states and cities has shifted in recent decades,especially since about 1980. There is more concentration of skilled workers in targeted areas, such as San Francisco and Boston, with consequential great rise in housing costs. Lower income people cannot afford housing as they might have in 1980. Low skilled immigrants migrate less to a highly valued metropolis. These trends increase disparity of wealth in America. I expect they contribute to the division of American between Blue and Red.
Go here and here.