Congressional districts, by density.
David H. Montgomery/CityLab

Close congressional races this November will likely hinge on the moods of suburban voters, a new CityLab analysis finds.

If you want to find a Republican member of Congress, head out into the country. To find a Democrat, your best shot is in a city. But to find a competitive election this fall? Head to the suburbs, where control of the House of Representatives will likely be decided.

More than 40 percent of the U.S. House of Representatives is composed of predominantly suburban districts, according to a new CityLab analysis that classifies all 435 U.S. House districts according to their densities. These seats are currently closely divided between Democrats and Republicans. But that balance could be washed away by a “blue wave” in November. There are 28 Republican-held suburban districts that are competitive1 this fall under FiveThirtyEight’s projections—close to 40 percent of Republicans’ 74 suburban seats. The number of suburban Democratic seats in play: 1 out of 90.

Our analysis shows that America’s electoral geography is more complex than a simple divide between “urban” and “rural” areas. There is a continuum of densities in the U.S., even within the category of “suburb.”

If dense districts usually give us Democrats, and far-flung rural districts usually go to Republicans, it’s the suburban places in between—less populous than left-leaning cities, but significantly denser than right-leaning rural areas—that will determine whether the GOP retains control of the House of Representatives.

The CityLab Congressional Density Index

To provide a new handle on the political geography of the upcoming midterm elections, we developed the CityLab Congressional Density Index, a model which classifies each congressional district by its mix of high- and low-density neighborhoods. (Read more about the model here). Congressional districts are made up of different types of places. A single district might contain cornfields, traditional suburbia, and skyscrapers. To capture this complexity, we calculated the density of every single neighborhood in each of the country’s 435 congressional districts, then grouped districts based on the different type of neighborhoods making them up.

Put your address into this interactive widget to see how we classified your district, then read on to learn more about the different categories and why this matters.

We identified six different types of congressional districts.  

Our analysis finds that the two middle categories of pure suburbs—sparse and dense suburbs—have the most competitive elections this fall.

Here is a map showing how CityLab classified each district. The purple-colored districts are predominantly suburban, and are the most likely to feature Republican incumbents in close 2018 races:

Here’s that same map, but with each district represented by a hexagram of equal size. This makes it easier to see many of the more urban districts, which take up far less land than rural districts but have similar numbers of people.

There are competitive races in all six categories of of districts, but the most suburban purple districts stand out. In those two categories, nearly 40 percent of all Republican-held seats are competitive.

The suburbs as battleground

The 2018 election will be decided in districts like Minnesota’s 3rd District, which covers a swath of affluent and well-educated Minneapolis suburbs. Republican Erik Paulsen has been easily elected every two years for a decade, but now is facing a tough fight in a district where voters backed both Barack Obama and Hillary Clinton in presidential elections.

“It’s always been a Republican district,” said Paul Anderson, a local state lawmaker who has managed Republican campaigns in the 3rd. “But it's also been known as a fiscally conservative, socially moderate (district)... You're talking about people that work for Cargill and Medtronic and General Mills and Best Buy and 3M.”

In dense suburban districts, Democrats already have the advantage of a majority of seats. But they are pressing forward in many more, launching credible challenges in 10 of of the Republicans’ 27 dense suburban districts. In sparse suburban districts, Republicans currently control more seats. But Democrats are poised to make headway there, too, with credible challenges in 19 of the 47 credible Republican districts.

Even in the 114 rural-suburban districts that contain a mix of rural areas and sparse suburbs, which are overwhelmingly Republican, Democrats could pick up 15 seats in districts seen as competitive.

If Democrats do well in November, they could turn their advantage in dense suburban districts into dominance, and their deficit in sparse suburbs into an advantage.

These patterns are in line with a growing body of research on America’s new political geography, nearly all of it applied to presidential elections.

In 2004, Bill Bishop famously called our attention to the “the big sort” where Americans organize themselves into neighborhoods and communities of similar socio-economic backgrounds, lifestyles and beliefs. We vote with our feet and choose the kinds of places we want to live, based not just on their housing options, school systems, amenities and tax rates, but based on the political attitudes and beliefs of the people who live there.

When Dante Chinni analyzed the 2012 election, he found that Democrat Barack Obama won comfortably in the innermost suburbs, while Republican Mitt Romney won big in exurbs. The areas in between, the so-called “middle suburbs,” were evenly divided. Ultimately, America’s big political sort has left medium-density suburban areas as the new political battlegrounds.

UCLA political scientist Jefferey Sellers similarly has found that suburban areas—particularly these in-between suburbs—have been the crucial swing areas of the last several presidential elections.

What makes a district swing

What are the factors that can help explain which suburban districts swing one way or another?

Much of it hinges on basic demographics. Take race, for example. Sparse suburban districts are on average more than 70 percent white. But dense suburban districts are just roughly half white. Both types of districts are on average more affluent and better-educated than purely rural or urban districts—but low-density suburban areas tend to be richer and have more people with college degrees.

The suburban districts where Republicans are endangered are also richer and better-educated than the suburban areas where Republicans are safe. Those competitive suburban seats have a median income $10,000 per year above the noncompetitive seats, and their rate of four-year college degrees is eight percentage points higher.

And gender factors in, too. “It's usually the suburban, college-educated white women in those districts that are putting them in play,” said Leah Askarinam, an analyst with Inside Elections. On the issues, she said, many of these voters might be moderate Republicans, but “they cannot tolerate the Republican Party under President Trump: the tweets, the chaos, the language.”

Of course, these suburban seats are not the only competitive races in the country. A handful of Republicans still represent districts with lots of urban neighborhoods—and eight of the nine are in serious danger of changing hands, per FiveThirtyEight’s ratings. Democrats are defending one competitive district here: Nevada’s 3rd District, an urban-suburban district where incumbent Representative Jackie Rosen is running for U.S. Senate.

Democrats are also trying to expand their small toehold in the country’s most rural districts. They currently hold 30 of the 184 pure rural or rural-suburban districts, but are competitive in another 20. Republicans are also making competitive threats to four Democratic-held rural districts.

But only thinking in terms of “urban” and “rural” won’t help us understand the outcome of this election.

“Democrats have a large enough pool where they can win some of the suburban districts, not all of them, (and) win some of the rural districts, not all of them,” adds Askarinam. “They only need to build up to 23 seats. There are dozens of seats that are competitive.”

Increasingly in America, density is our political destiny, with suburbs playing the crucial role not just in who is elected president but also who controls Congress. In 2018, more than just about any other year, those two go hand in hand.

In the coming weeks, we’ll delve further into this data to explore America’s density-based political divide. But you can explore it, too. Check out our methodology here and explore the full CityLab Congressional Density Index. Send us your feedback and analysis.

  1. Districts are defined as competitive where either party has at least a 25 percent chance to win under FiveThirtyEight’s projections, or where one party has more than a 75 percent chance to capture a district held by the other party.

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