Agreed with a few others in this thread that it's best to consider SD collectively with ND, NE, and KS, or at the very least with ND. Growing up in the Northeast, to me it always seemed like these low-population red states always belonged more in the West than in the Midwest.
But now I tend to think they belong in the Midwest because I see the 100th meridian as the best line to divide the U.S. into eastern and western halves, and most of the population of these states lives east of the 100th meridian. You can kind of see the dividing line in the
2020 presidential election county map; the western halves of the states are a mix of dark red rural areas and blue Indian reservations, while the eastern halves have slightly less dark-red rural areas combined with pink and some blue counties in metro areas.