Trump's underperformance in the plain states is likely due to his massive decline with white college educated voters when republicans tend to do very well with such voters in these states. Iowa has a larger population of voters without college degrees, counteracting and overcoming such a decline with college educated voters.
As usual, college-educated people tend to reject demagogues, and this time the demagogue is on the Right side of the political spectrum (except on foreign policy). They have paid attention to formal logic, typically from their high-school geometry course, they are capable of detecting contradictions, and they have some historical perspective. Demagogues invariably make reckless promises that ill-educated people can rarely reject for reasons other than ideology. Well-educated people can reject those contradictions because they pay attention.
The Plains states from Kansas to North Dakota have high educational standards (which distinguishes Kansas from neighboring Oklahoma, where Donald Trump is doing well) Hillary Clinton cannot win these states on her own; except for 1964, none of these states (exception: NE-02, largely Greater Omaha) has gone for any Democratic nominee for President since the 46-state landslide by FDR. Donald Trump can lose one or more of these conservative-but-not crazy states.
