Detroit was limited from the day it was founded and it had nothing to do with Democrat vs Republican. It inevitably had to shrink in the size of industry and population that be supported there.
Detroit is an inland port and once the size of ships increased to where they could not traverse the locks and canals of the St. Lawrence seaway to the Atlantic Ocean, the city, like the other Great Lakes cities both American and Canadian, could not compete in manufacturing with other locations around the world.
The costs to widen parts of the seaway and excavate it deeper, as well as replace the locks, would never be recouped.
San Francisco is in the same position. The port is too small for the current generation of ships. Instead, the ships port in Long Beach and Seattle/Tacoma. (Ships exceeded the size of the Columbia River to Portland, Oregon in 2015).