In 1968 Humphrey loses NC, IN, ND, SD, NE, KS, OK, MT, WY, ID, UT and AK. No Democrat has won these states since (some of them won them in their first run, but lost them in their second).
I'm not sure what the connection is, but it seems that Humphrey failed to connect with a lot of quiet, rural states, which I guess bought into Nixon's "law and order" and were not influenced much culturally by the counterculture movements of the '60s that changed politics in urban centers in later decades. Could that be what makes these states so hostile to the Democratic party to this day?
Most of these have never been very friendly to Democrats. Before the 1964 Johnson landslide, the non-Southern states had at least three times in a row voted Republican (including voting for Nixon in 1960). ND, SD, NE, KS and IN also voted R in 1948, 1944 and 1940.
If there was any Democrat that could connect to these "quiet, rural states" , it was probably Humphrey, who grew up in rural South Dakota and started his political career from Minnesota. In fact, South Dakota and Indiana were among the few non-southern states that trended to him.
My guess is that the 'hostility' against Democrats started with Woodrow Wilson "he kept us out of war" in 1916, and the maps below show you, why that could have been so:
1990 census:
Remember that most of the German immigration into the Midwest took place in the late 19th century, so for these immigrants and their children, WW I literally meant fighting their brothers or cousins. They voted for Wilson hoping WW I participation could be avoided, and found themselves deceived.
Herbert Hoover as first German-American President, with his roots in the Midwest, and subsequently Pennsylvanian Dutch, Kansas-based Dwight D. Eisenhower, then cemented the loyalty to Republicans.
