There’s no connection to Democrap leaders, mayors, unions etc...
All of these are huge factors, but there’s more to it than just poor governance. I think a big part of the decline of these old “Rust Belt” cities is that a location on the Great Lakes system is no longer as important as it used to be for industrial activity. Access to ports on the East and West Coasts, along with access to major Class I railroads, have become bigger factors in industrial site selection over the last few decades.