Your assessment of Montgomery County sounds right.
My impressions of the Main Line were based on the people I met and knew there, all of whom happened to be white and Democrat and most of whom were either Catholic or Jewish. That fits your description. More than likely, I just never came across the majority Republicans there. Or maybe all those supposed Dems were voting GOP secretly...
When I moved 20 years ago, the Dems were just beginning to take control of the county seat, Norristown, where, up into the ‘80’s, mostly everyone registered Republican because the courthouse hired only Republicans. I always figured the local Republicans were responsible for that policy, but maybe the policy was county-wide... I don’t know.
Norristown, though, is not a recent victim of urban blight. It turned into the pits almost 30 years ago. Over the last 15 or so years, it has improved dramatically. Crime is high, but revitalization has been ongoing, and builders have been buying land and investing in it, so it may turn around yet. There’s a large population of illegal immigrants there now, though.
I know little about NJ and its people, even after 11 years. I don’t feel much loyalty to this state, either. All I know is the Dems have so much power over our town that the local GOP doesn’t even bother putting anyone on the ballot anymore.
Correction: almost 40 years ago. I keep forgetting how old I am.