North Dakota vs. Minnesota

[FerrisBueller86]

In the early 20th century, both North Dakota and Minnesota were heavily Republican.  In the 2nd half of the 20th century, North Dakota remained heavily Republican while Minnesota became one of the most heavily Democratic states.  The Democratic presidential nominee has carried Minnesota in every election starting with 1960 except for 1972.  Minnesota is the only state never carried by Reagan.  During this same period, the Republican presidential nominee has carried North Dakota in every election except for 1964.

What made North Dakota and Minnesota vote together for most elections from the late 1800s to the 1930s?  What made these two states diverge?  One would expect both states to be similar.  Both are very cold places (not good states for starting a nudist colony), and both have a large Scandinavian American population.