I’ve always felt like it’s Illinois and Indiana. I know we border Kentucky, but it’s a small border on the southern tip, so there’s not a lot of interaction between folks in the two states. Illinois and Indiana are connected in many ways and represent very different politics.
I can also see good arguments for Washington/Idaho, Maryland/WV, and Minnesota/ND, but like IL/KY, I feel like population centers of those states aren’t as connected as IL and IN are.
Ehh, isn't Illinois just basically (politically speaking) Indiana with a massive city added to it? I don't feel like crossing the Illinois-Indiana border results in a significant sudden cultural difference. I'd look for state lines where there are significant cultural trend changes basically exactly on the state line. An underrated possibility might be Missouri and Iowa, although this is gradually decreasing. Looking at a religion map shows that, like much of the South, Baptists dominate Missouri right up to the border with Iowa, but they have almost no presence right across the border into Iowa. Likewise, historically, the rural Northern Missouri counties have voted substantially to the right of the rural Southern Iowa ones.
Florida-Georgia is another possibility. I once saw a map of Cuban ancestry, and the Florida-Georgia border was plainly evident. Add that to the number of retirees and vacation communities in North Florida, and it's actually decently different from South Georgia.
Politically, Massachusetts-New Hampshire is another good option, especially with the exurbs of Boston in Southeastern New Hampshire typically voting to the right of the state.
