Metropolitan political divides have replaced the old red-state/blue-state cleavage.

American politics turn on a now familiar set of categories: red states vs. blue states, rich states vs. poor states, Frostbelt vs. Sunbelt. But these generalizations mask deeper, less visible fissures in our political geography.

We have written a great deal about the role of density in metropolitan voting patterns, highlighting the remarkably consistent and robust political red-to-blue tipping point that occurs when a metro reaches a density of roughly 800 residents per square mile. I took a deeper look at our emerging political geography in a recent feature for Politico magazine, where I argued that the suburbs have become the key turf in American politics today.

The older, denser suburbs outside our central cities have emerged as the major points of political cleavage in America, the places where Presidential elections are won or lost. "The key political fissure in American politics no longer runs across the country's swing states," I explained, "but zigzags through the rapidly growing ranks of what I call its 'distress 'burbs.'"

I drew on the detailed research of USC political scientist Jefferey M. Sellers, including important new data that he and his colleagues put together in The Political Ecology of the Metropolis, released this summer. Sellers has compiled a massive time-series data set on the metropolitanization of American politics, using information on local voting patterns across America and comparing that to a slew of economic and demographic indicators. His fascinating findings are worth a closer look.

He argues that the massing of both population and economic activity in metropolitan areas is injecting strongly local influences into national politics. "Metropolitanisa­tion," he concludes, "is contributing to a re-territorialisation of politics."

But it's not so much the urban centers that have come to shape our politics. The suburbs – where the majority of the electorate lives – are the central battleground of American politics.

Sellers and his colleagues analyze the political characteristics of cities and suburbs across many advanced nations. Sellers's own chapter covers the U.S., and it includes some eye-opening insights. While most previous research has looked mainly at states and counties, Sellers has developed a detailed data set on the municipalities that make up America's metro regions. He tracks the political geography of the 1996, 2000 and 2004 elections across twelve U.S. metros with populations of at least 450,000: New York, Los Angeles, Philadelphia, Detroit, Atlanta, Seattle, Cincinnati, Fresno, Birmingham, Syracuse, Wichita, and Kalamazoo.

Democrats have a "decisive" advantage in dense, urban localities and poorer, majority-minority suburbs. In the affluent suburbs, Sellers explains, "Republicans enjoy an analogous, if less dramatic" advantage. He notes that "a pervasive divide separates the Republican low density areas of metropolitan peripheries from the Democratic urban centres and minority suburbs." At the broad metropolitan level, votes follow the same red/blue, rich/poor pattern identified by Larry Bartels and Andrew Gelman at the state level. Sellers found that municipalities with educated and affluent voters tended to vote with their state's winners – they voted more Republican in red states and more Democratic in blue states.

With these bases locked down, the key political footballs – the new "swing states," so to speak – are the swelling ranks of economically distressed suburbs, where poverty has been growing and where the economic crisis hit especially hard. There are now more poor people living in America's suburbs than its center cities, and as a recent Brookings Institution report found, both Republican and Democratic districts have been affected by this reality.

To get a better idea of how this works, take a look at the figure below, which tracks the gap between votes for Republicans and Democrats by type of town between 1996 and 2004. At the far right, affluent and low-density suburbs both went largely for Republican candidates. There was an analogous Democratic advantage in urban concentrations and poor minority suburbs, at left.

It's the distress 'burbs – poor non-minority and the middle-class suburbs – in the middle of the graph, with a near even split in votes for Republican and Democrat candidates.

Figure 4a. Partisanship (Percent Republican minus percent Democrat) by municipality type, 1996-2004.

The distress 'burbs are the places most likely to shift between parties. And here, Sellers identifies another clear pattern. Struggling communities in red states tend to lean left, while disadvantaged suburbs in blue states increasingly vote for the right. As he concludes:

In red states as well as some of the blue state metropolitan areas, the more affluent suburbs endorse the majority preferences of the wider region. A similar parallel between the regions marks metropolitan communities with more socioeconomic hardship. The most disadvantaged metropolitan communities of the red states have diverged most from the Republican preferences of the wider region. The most disadvantaged communities of the blue states have responded more than other communities there to Republican appeals. 

America's new metropolitan geography is overlaid by one additional factor: voter participation. Turnout levels have ranged between 52 and 62 percent over the past several national elections. Even though Democrats have the clear advantage in raw numbers, Republicans dominate the kinds of communities where people are more likely to actually vote. Turnout, Sellers finds, tends to be higher in GOP strongholds – the more affluent, highly educated suburbs and low-density rural and exurban areas, all places with higher levels of home-ownership.

Democrats face an uphill battle in urban neighborhoods and minority-majority suburbs closer to the urban center. "Turnout in the Democratic urban and minority strongholds, and shifts among middle class and distressed suburbs are the decisive swing factors in U.S. national elections," Sellers wrote in an email.

In more recent political contests, this dramatic battle for the suburbs has led to some seemingly surprising electoral shifts. From 1996 to 2004, the middle-class suburbs leaned Republican. Since then, it's been a tug of war, with the Democrats coming out on top in 2006-8, the Republicans pulling ahead in 2010, and the Democrats making a comeback in the 2012 election.

But while voting patterns in the distress 'burbs shift back and forth, ongoing urbanization and increased density appear to favor the Democrats. Regardless, the battle for the suburbs remains fierce, and they have come to define America’s new electoral map. 

Top Image: Yard signs for Obama/Biden in Florida in 2008. Courtesy Flickr user shutterblog.

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