I would agree that urban New England is heavily Roman Catholic, but the rural areas are largely Congregationalist. I would also have included the Mennonite/Amish areas of Pennsylvania and Ohio. Alaska has historically been Russian Orthodox, but I don't know if that's true today.
Rurual New England hasn't been "largely Congregationalist" for 150 years. New England has very few New English any more. Its mostly, Irish, French-Canadian, Portuguese, and Italian, hence over 50% Catholic nearly everywhere.
The New English forgot to have children starting about 100-150 years ago. Its been downhill for them ever since.