North Dakota vs. Minnesota

[WI_Dem]

I think it has something to do with the urbanization of MN versus ND and also a general shift in political philosophy as a result of the Great Depression. The same is true to a lesser extent in other Great Lakes states that voted heavily Republican earlier and then became at least swing states as a result of the Great Depression. For whatever reason, even though ND and some of the other plains states voted for FDR in '32 and '36, they flipped on him and almost never came back to the Democratic Party until '64 and haven't voted Democratic since then either.

[WI_Dem]

They are nothing alike now, but the original poster's point was what made them change from their similar voting patterns in the early part of the 20th century to the later part.