There are a lot of midsize Rust Belt cities to watch for this. For example, it's pretty easy to imagine a world where St. Louis County, MN is safe R in the 2030's, also Onondaga/Monroe/Erie in NY.
Monroe+Onondaga counties have shifted in the opposite direction from Erie County since the Bush years, especially in Monroe County. Republicans are even losing strength downballot there as well, with the county executive seat flipping to Democrats last year for the first time in almost 30 years and only the second time ever. It is not at all a sure bet that those three will all be voting reliably Republican by next decade.
