The east-west polarisation is pretty striking, down no doubt to the fact that that most states in the west only have a small percentage of their populations living in small towns and small towns have trended hard towards the GOP over the past 20 years. The other key driving force seems to be the presence of large metros which have become ever more democratic. As this article shows,
https://www.realclearpolitics.com/articles/2017/01/20/how_trump_won_--_conclusions_132846.html metros with more then 5 million people have consistently been shifting more democratic, this particularly shows in states like Texas and Illinois.
Overall I added up the states, 30 states with 289 EV's have trended GOP so the trends if they were to continue indefinitely which they won't of course are more favourable for the Republicans, especially in the senate.
