Missing the point: cities, cosmopolitanism, and progressivism as a multicentury trend
People have been writing about the link between "sidewalks" and votes for Democratic candidates, or the support for "liberalism" in cities and towns versus the support for conservatism is less developed areas. See "Why Do Sidewalks Predict Whom We’ll Vote For?" from Streetsblog.
Um, this has been the case for thousands of years, although back in the day, urban middle classes (bourgeoisie) in countries like France weren't looking to vote for President Obama. (And of course, I wrote about this as a more recent phenomenon as it related to the 2004 article from the Stranger.)
The history of democracy is in part a history of urbanism.
-- "Rise of Towns," International History Project
-- "History of World Civilization - Greek Civilization: Polis; Athens" from the Historian and his facts blog
The rise of democratic government grew out of the creation of towns as semi-independent self-governing communities not directly managed by the monarchy.
Back in college, I remember coming across the work of Prof. Ronald Inglehart at the U of Michigan and his study of post-industrial social and political attitudes in Europe (although I never took a course from him). Now this work has been repositioned and expanded into a international political attitudes survey project called the World Values Survey.
-- Chapter on "Culture and Democracy" by Ronald Inglehart
-- "Modernization, Cultural Change, and the Persistence of Traditional Values," Inglehart and Baker, American Sociological Review, 2000
The real issue isn't why urban-metropolitan area residents are progressive--and it's all relative. In the DC metropolitan area, places like Prince William and Loudoun Counties in Virginia are considered quite conservative relative to Arlington or Montgomery Counties, DC or incorporated cities like Takoma Park, Maryland. But compared to rural Montana, they are decidedly liberal, and voted for Obama.
The real issue is why exurban residents are neither "progressive" nor "post-industrial" although this is by no means a strictly urban-suburban-rural continuum.
And this is discussed in thousands and thousands and thousands of histories on the development of cities, modernization, and political theory.