Illinois Congressional Districts
Illinois has 17 congressional districts. We’ve generated 5,000 sets of randomly simulated districts according to the relevant criteria. Three of these plans are shown here, along with the actual enacted map.
Partisan Features
In Illinois, Democrats win about 60% of the vote in a typical statewide election. Proportionally, that would translate to 10.1 Democratic seats out of 17 total.
But proportionality isn’t guaranteed, even in a fair redistricting process. In our simulated plans, Democrats won anywhere from 11.1 to 12.6 seats on average, with 11.9 being the most typical. In contrast, we expect the enacted plan to yield 14.3 Democratic seats on average, which is more than 100% of all simulated plans.
The graph below breaks this down in detail, showing how each district of the enacted plan compares to the set of simulated districts.
Gerrymandering metrics
There are other ways of measuring the partisan skew of redistricting plan. The graph below shows two these metrics. The deviation from partisan symmetry measures the expected difference in each party’s share of seats if they each won 50% of the statewide vote. The efficiency gap is calculated as the difference in the number of wasted votes for each party.
Learn more about these metrics here.
Traditional redistricting criteria
Factors other than partisanship are important for redistricting, too. The graph below shows the geographic compactness and the number of counties which are split into multiple districts. As far as compactness, Illinois’s enacted plan scores a 0.14, less than 100% of all simulated plans (a higher score means more compact). It splits 32 counties, compared to an average of 9 counties for our simulated plans.
Political Geography
These two maps show the partisan lean across Illinois in a typical statewide election, and the share of minority voters around the state.
More information
Download the data for Illinois.
Learn more about our methodology.
Elections included in analysis:
- President 2016
- US Senate 2016
- Governor 2018
- Attorney General 2018
- Secretary of State 2018
- President 2020
- US Senate 2020
Redistricting requirements
Our algorithmic constraints used in simulations are in part are based on some of these requirements and discretionary criteria. See full documentation and code for the complete implementation details.
In Illinois, districts must, under Ill. Const. Art. IV, § 3:
- be contiguous
- have equal populations
- be geographically compact