In 2008 Obama did really well in parts of the rural midwest with residual statewide party strength. The South Dakota Dems and the D-NPL still had residual appeal in rural Scandinavian areas of the state (3/4 of Dakota Senators were Democratic in 2008).
E.g., here's a Swedish ancestry map (source: Wikimedia)
Western parts of these states are sparser and most industry is ranching (before the oil boom at least) so liberal politics were never going to be appealing there. But east of the Missouri you see a lot of Scandinavian farmers.
(note that NE corner also has a reservation)
(Heitkamp basically
won all of these Eastern counties in 2012 and even won
most of them in 2018)
for reference here's the Minnesota map in 2008 - similarly Obama had a lot of strength in western and southeastern parts of the state that are heavily Scandinavian (and also coterminous with the areas he won in the Dakotas)
The bottom fell out in all of these places with the oil boom and general urban/rural polarization.
