Geographic Region: Congressional Districts

Congressional districts are the 435 areas from which people are elected to the U.S. House of Representatives. After the apportionment of congressional seats among the states based on decennial census population counts, each state with multiple seats is responsible for establishing congressional districts for the purpose of electing representatives. Each congressional district is to be as equal in population to all other congressional districts in a state as practicable.

The founders designed the U.S. House of Representatives to represent the people rather than the states. Article I, Section II of the Constitution provides each state at least one U.S. representative, while the size of a state’s delegation to the House depends on its total population. With the aim of dividing representation among the states proportionately, Congress based apportionments on changes in state population as recorded in each decennial census since 1790. To keep the House at a manageable number, Congress set the size of the House at 435 voting members.

Almost all site-based data (e.g., child care) and almost all demographic data (e.g., population) are presented by congressional district in IECAM. Non-site-based data (e.g., early intervention) are not presented by congressional district.

Source of definition:

• U.S. Census Bureau, Geography Division. Geographic Terms and Concepts – Congressional District
• Office of the Clerk, U.S. House of Representatives
• House History: Congressional Apportionment

Below find the two most recent (117th and 118th) U.S. Congressional District maps for Illinois. Each session lasts for two years, corresponding to the 2-year terms of the elected representatives. 

• 118th Congress: 2023 and 2024
• 117th Congress: 2021 and 2022

After each decennial census conducted by the U.S. Census Bureau, the U.S. congressional districts for each state are redrawn. The new districts are the basis for the next general election, and they take effect in the U.S. Congress after that election. (For example, the latest decennial census was conducted in 2020. The new districts were redrawn in 2021, and they were used as the basis for the general election in November 2022. They took effect with the beginning of the subsequent 118th U.S. Congress in January 2023.)

Based on the reapportionment that was conducted after the 2000 decennial census, Illinois was apportioned 19 congressional districts. After the redrawing of the district boundaries by the Illinois legislature in 2001, the new districts took effect with the 108th Congress in January 2003. Neither the number nor the boundaries of congressional districts change until the next decennial census.

After the 2010 decennial census, both the number and boundaries of the congressional districts changed. Based on the reapportionment, Illinois was apportioned 18 congressional districts (a loss of one district). The state legislature redrew the congressional district boundaries in 2011, the new districts served as the basis of the 2012 general election, and the new districts took effect with the 113th Congress in 2013.

After the 2020 Census, Illinois was apportioned 17 congressional districts, a loss of one district. The state legislature has redrawn the congressional district boundaries, and they served as the basis of the 2022 general election. 

Comparing congressional districts across decennial censuses

In IECAM’s online database and multiyear search feature, the congressional districts up through 2012 (the last year of districts based on the 2000 decennial census) will be considered one region type, the congressional districts beginning in 2013 (the first year of districts based on the 2010 decennial census) will be considered another region type, and the congressional districts beginning in 2023 (the first year of districts based on the 2020 decennial census) will be considered yet another region type (when the data for that year becomes available).  

Because these region types have different numbers of districts (19 for the post-2000 census; 18 for the post-2010 census; 17 for the post-2020 census) and because the districts cover different geographic areas, it is not possible to compare data for post-2000, post-2010, and post-2020 census districts. For example, you cannot compare data on PFA capacity or population of children for Congressional District 15 in 2010 with the data on PFA capacity or population of children for Congressional District 15 in 2014. Such a comparison would be misleading.

However, you are able to compare data for different years that apply to the post-2000 census districts and for different years that apply to the post-2010 and post-2020 census districts. For example, you can compare data on PFA capacity or population of children for Congressional District 15 in 2010 and in 2011, and you can compare the same data for Congressional District 15 in 2023 and in 2024 when those data become available.