This ^

It's mostly urban vs rural, even on the coasts and in states like CA.

Visual: https://brilliantmaps.com/2016-county-election-map/

Inside vs outside of the city and wealthy inner and middle suburbs is a far more massive cultural divide than red state vs blue state, nominal political party membership, rich vs poor or anything else.

From my own personal, subjective experiences, I've found this to be the big one. I've lived in 8 states now, in all regions except the Northeast, from big cities to smaller towns (SF to towns of ~30,000) and the rural vs. urban divide seems to be the most pronounced to me.