The comfortable victories of Barack Obama in two successive presidential elections in 2008 and 2012 were forged by an alliance of white voters in the north and minority voters across the country. The contribution of white voters was critical to the performance of Obama. This was reflected in the way he won extremely white and rural Northern states like Iowa, Maine, New Hampshire and Vermont, as well as Northern states with a mix of urban and rural population, like Wisconsin, Ohio, Michigan, Minnesota and Pennsylvania.
In 2016, the support of rural voters (who tend to be overwhelmingly white) for the Democratic Party collapsed as Hillary Clinton managed to lose even light blue states like Wisconsin, Pennsylvania, Michigan and was blanked out in Ohio and Iowa. She even close came to losing in New Hampshire and Minnesota. Vast number of white, rural counties in Middle America, which voted for Obama in 2008 and 2012, turned their backs to the Democratic Party and instead voted en masse for Donald Trump. This was compensated, to some extent, by the gains she made in the heavily urban states and territories of Texas, California, Arizona, Massachusetts and District of Columbia. Unfortunately for Clinton, the gains did not prove meaningful at the electoral college level, as California, DC and Massachusetts were anyway going to vote for Clinton while the gains made in Texas and Arizona were not sufficient to deliver the states to her.
The relationship between the percentage of total votes obtained by Hillary Clinton in a particular state to the level of urbanization of the state has been shown in the following chart:
In USA, at the country level, the percentage of population who live in urban areas is 80%. As many as 33 states have level of urbanization below the national average. Clinton lost in 27 of them. In contrast, Obama had lost only 22 of them in 2012. Among the states and territories that are more urbanized than the country as a whole, Clinton ended up losing only Texas, Arizona and Florida.
However, it is not just that the vote share of Clinton was higher among more urbanized states. She also gained votes in more urban states and lost votes in more rural states. This is reflected in the following chart which plots the percentage gain in margin by Clinton in a particular state vs the level of urbanization of the state. (By percentage gain in margin, I mean how the margin changed in 2016 from the level of 2012; for example, if Obama won a state by 3% in 2012 and Clinton won the state by 5%, the percentage gain for Clinton would be 2%. Also note that Utah has been excluded from all the charts because of the strong performance of third party candidate McMullin on the ballot there).
All the states where Clinton was able to improve on the performance of Obama from four years back had at least 70% of the respective population living in urban areas.
This relationship also holds good if I just restrict the level of urbanization to the percentage of population which live in cities with a population of more than a hundred thousand.
Thus, the urban-rural divide which was already present in the American politics has become even starker in the 2016 election. If this trend holds, Iowa will become a red state while the margins of the Democrats may further shrink in Maine, Vermont and New Hampshire. Most states of the Rust Belt and Mid West will continue to remain swing states while Democrats may gradually improve on their performance in Texas and Arizona.