Why is North Dakota more Republican than South Dakota?

[Schiff for Senate]

ND also has a greater share of whites and a smaller share of Native Americans compared to SD.

Good point.

Just look at ND and SD's Native American counties. SD has more Native American counties, and these counties vote slightly more liberally than ND's such counties overall.

Rolette and Sioux Counties, ND's only blue ones, gave Biden 65% and 68%, respectively (average: 66.5%). In contrast, SD had six blue counties, five of them majority-Native-American, two that went Democratic with over 70% of the vote; the five majority-Native-American counties gave Biden 64%, 58%, 88% (Oglala Lakota was one of the bluest counties nationally), 77% and 53% (average: 68%).

[Schiff for Senate]

ND also has a greater share of whites and a smaller share of Native Americans compared to SD.

More of them are also of German descent, and German-Americans are these days one of the most strongly Republican voting blocs. People act like Germans were all completely assimilated and thus there is no distinction between how they and other white Americans vote today, but that's not quite true. It may not be as noticeable as it was back in the day, but German-Americans still have a clear Republican lean and that alone handed Trump the presidency in 2016.

What do you suspect is the cause of this?