It’s urban vs everything else.
Maybe not even that.
Deep State shoved RCV down NYC’s gullet for a reason.
If NYC voters are so reliably lib, why bother...
It typically is urban vs. rural.
The other blue/red are college towns. Look at the map for WA state for an example. The Blue country in Eastern WA are Spokane and to the south is the county where Washington State is located.
The same in every other place where big universities are located around the country. Here in NH the bluest part of the state is where Dartmouth in located. The next bluest is where UNH is located.