Counties are the familiar building blocks of American civic life: Their shapes tend toward the reasonably regular, and they very rarely change over time, making it fairly easy for most folks to pick out their home county on a map.
Pennsylvania map showing location of PA-17 and PA-SD-37
Congressional districts, on the other hand, are a different beast entirely. Often gerrymandered into unrecognizable twists and turns, they must almost always be redrawn every decade, and sometimes more often. Even if you know which district you live in, and even if you have a sense of its shape, it’s surpassingly difficult to piece together how your county (and its neighbors) fit into the electoral landscape.
But there are many good reasons to want that sort of information. Lots of local political parties and activist groups are organized at the county level, for instance, but how much weight does each have in a particular congressional district? Another example: Let’s say your county executive decides to run for Congress. Does she represent the bulk of the district she’s now seeking, or only a small sliver?
To help make sense of this thorny problem—and several related ones—Daily Kos Elections is pleased to offer a new data set that precisely details the overlap between every single county in America and all 435 congressional districts.
But that’s not all. We’ve also calculated similar data for every state illuminating the relationships between:
Below, we’ll walk you through how these spreadsheets work, illustrated with some specific examples.
To begin with, each of these spreadsheets works going in either direction; for instance, if you’re curious about a particular congressional district, you can see what percentage of the district is made up of residents of each component county. But you can also look at the breakdown of each county to see what percentage of each county’s residents live in which congressional district. (These spreadsheets all rely on 2010 population statistics; in other words, the figures from the most recent census, which were used in the last round of redistricting.)
Let’s take a look at Pennsylvania’s 37th State Senate District, which was the site of an important special election that saw Democrat Pam Iovino flip a seat from Republicans earlier this year. Using the county/legislative district sheet and flipping to the tab labeled “PA,” we can see that:
- 242,336 people who live in SD-37 live in Allegheny County, or 92.0% of the district.
- 21,213 people who live in SD-37 live in Washington County, or 8.0% of the district.
Conversely:
- 242,336 people who live in Allegheny County live in SD-37, or 19.8% of the county.
- 21,213 people who live Washington County live in SD-37, or 10.2% of the county.
The bulk of SD-37, therefore, comes from Allegheny, but the reverse isn’t the case: Most residents of the county don’t live in SD-37. In fact, Allegheny County, which is home to Pittsburgh and 1.2 million people, is rather evenly divided among no fewer than four other state Senate districts, in addition to the 37th:
- 254,885 people who live in Allegheny County live in SD-38, or 20.8% of the county.
- 261,773 people who live in Allegheny County live in SD-42, or 21.4% of the county.
- 252,278 people who live in Allegheny County live in SD-43, or 20.6% of the county.
- 212,076 people who live in Allegheny County live in SD-45, or 17.3% of the county.
Now perhaps one day Iovino, a Navy veteran and former VA official, might seek a promotion to the U.S. House. But which district might she run in? It turns out that her state Senate district overlaps with no fewer than three different congressional districts:
- 21,213 people who live in SD-37 live in PA-14, or 8.0% of the SD.
- 128,593 people who live in SD-37 live in PA-17, or 48.8% of the SD.
- 113,743 people who live in SD-37 live in PA-18, or 43.2% of the SD.
Again, conversely:
- 21,213 people who live in PA-14 live in SD-37, or 3.0% of the CD.
- 128,593 people who live in PA-17 live in SD-37, or 18.2% of the CD.
- 113,743 people who live in PA-18 live in SD-37, or 16.1% of the CD.
You’ll notice there isn’t much overlap between SD-37 and PA-14, which is the one Republican-held district among this trio. That’s interesting because SD-37 became vacant in the first place because Republican Guy Reschenthaler won election last year to PA-14, a dark-red seat. Evidently, the fact that he represented only 3% of PA-14 while in the state Senate wasn’t an insurmountable obstacle.
Iovino, however, doesn’t live in PA-14; rather, her hometown of Mt. Lebanon in the Pittsburgh suburbs is located in PA-17. That district is represented by Democrat Conor Lamb, a rising star who could very well run for statewide office one day. (PA-18, meanwhile, is a safely blue district centered on Pittsburgh, which has been held by Democrat Mike Doyle since 1995; it’s possible, though, that the seat could come open with a Doyle retirement at some point in the next decade.)
So let’s suppose Lamb does run for Senate, say, and Iovino decides to seek PA-17, which would be an open seat in this scenario. Iovino’s SD-37 would in turn also become open, presenting an opportunity for local politicians to move up to her post. Some of the most eager would almost surely be members of the state House whose districts overlap with SD-37. Examining our spreadsheet that analyzes the relationship between upper-chamber and lower-chamber districts, we can see that there are quite a few contenders:
- 2,852 people who live in SD-37 live in HD-16, or 1.1% of the SD.
- 455 people who live in SD-37 live in HD-27, or 0.2% of the SD.
- 27,607 people who live in SD-37 live in HD-38, or 10.5% of the SD.
- 29,763 people who live in SD-37 live in HD-39, or 11.3% of the SD.
- 61,632 people who live in SD-37 live in HD-40, or 23.4% of the SD.
- 33,137 people who live in SD-37 live in HD-42, or 12.6% of the SD.
- 61,658 people who live in SD-37 live in HD-44, or 23.4% of the SD.
- 17,959 people who live in SD-37 live in HD-45, or 6.8% of the SD.
- 28,486 people who live in SD-37 live in HD-46, or 10.8% of the SD.
Conversely:
- 2,852 people who live in HD-16 live in SD-37, or 4.6% of the HD.
- 455 people who live in HD-27 live in SD-37, or 0.8% of the HD.
- 27,607 people who live in HD-38 live in SD-37, or 43.1% of the HD.
- 29,763 people who live in HD-39 live in SD-37, or 49.4% of the HD.
- 61,632 people who live in HD-40 live in SD-37, or 100% of the HD.
- 33,137 people who live in HD-42 live in SD-37, or 54.5% of the HD,
- 61,658 people who live in HD-44 live in SD-37, or 100% of the HD.
- 17,959 people who live in HD-45 live in SD-37, or 29.4% of the HD.
- 28,486 people who live in HD-46 live in SD-37 or 45% of the HD.
For the purposes of state representatives hoping to get elevated to the Senate, the first set of bullet points is likely to be the most meaningful. Two legislators (both Republicans) represent almost a quarter of SD-37: Natalie Mihalek in HD-40 and Valerie Gaydos in HD-44, potentially giving either a leg up. The Democrat with the biggest slice of SD-37, meanwhile, is Dan Miller in HD-42.
And this brings us back to where we started: the relationship between counties and congressional districts. We’ll conclude with a look at PA-17, which served as the central domino in our series of examples above:
- 517,660 people who live in PA-17 live in Allegheny County, or 73.4% of the CD.
- 170,539 people who live in PA-17 live in Beaver County, or 24.2% of the CD.
- 17,489 people who live in PA-17 live in Butler County, or 2.5% of the CD.
And one last time, conversely:
- 517,660 people who live in Allegheny County live in PA-17, or 42.3% of the county.
- 170,539 people who live in Beaver County live in PA-17, or 100% of the county.
- 17,489 people who live in Butler County live in PA-17, or 9.5% of the county.
Allegheny County, given its size, is the center of gravity in PA-17, but note that the entirety of Beaver County (which is the 19th-largest of Pennsylvania’s 67 counties) is located in the district.
We hope that our new data sets help unlock the relationships between counties and election districts, and that the next time a state legislator anywhere in the country looks for a promotion, you’ll know exactly where they stand vis-a-vis the new constituents they’re hoping to represent.
A brief methodological note: If you’re wondering how we amassed this information, we used the Missouri Census Data Center’s Geographic Correspondence Engine. The GCE is fairly simple-to-use site that uses Census Bureau data and can return a detailed list of how two different categories of geographies (anything from cities and counties to school districts or census tracts) overlap.
UPDATE: We’ve also added a new set of correspondences for nine states in the Northeast where geographic divisions below the county level are more commonly used as reference points. This is particularly true in New England, where several states have mostly or entirely eliminated county governments and are instead administered at the town level. This new data set includes Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, and Vermont.
