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Redistricting Basics

Answers to fundamental questions about the redistricting process and how to draw and analyze maps

What is redistricting?

Redistricting is the process of redrawing electoral boundaries. It occurs for all levels of representative government, including the US House of Representatives, state legislatures, county and city government, schools districts, and more. These boundaries define the constituency that will be represented in government and the district in which a candidate for office runs.

What is gerrymandering?

Gerrymandering is the drawing of electoral boundaries for unfair electoral gain. The most common forms of gerrymandering are political, racial, prison-based, and incumbent-based. Political gerrymandering occurs when boundaries are drawn to provide one party an electoral advantage over another. Racial gerrymandering occurs when boundaries are drawn to dilute the voting power of minority groups, or when boundaries are drawn predominantly on the basis of race. Prison-based gerrymandering occurs when incarcerated individuals are counted at their facility of confinement for the purposes of redistricting, rather than at their home address. Incumbent based-gerrymandering occurs when boundaries are drawn to maximize the chances an incumbent is re-elected, by retaining the incumbent’s home and core constituency in the district, or ensuring other incumbents or challengers are drawn outside of the district.

How do I draw a legally compliant map?

In order to draw legally compliant maps or identify gerrymandering, you need good data, a mapping tool, and an understanding of the criteria in your state. We host all the data you need to draw or analyze maps, and have articles for each dataset so you can learn more about them. We also have a help desk that can answer any questions you may have about the data or how to use it. All our data comes with metadata documenting sources and methodology, and our Partner Data Validation project ensures that our precinct boundaries joined with election results are accurate and reproducible. Once you register for an account, you can download datasets individually or register as an API user to download multiple files at once.

Once you have the data, you will need to choose a mapping tool. There are free tools available that can be used to draw or analyze maps, and we have partnered with many of them to demonstrate how to draw or analyze district or community of interest maps. If you are using more than one free mapping tool at once, you can import and export files across them to make the most of their different features. You can also find out which mapping software is being used by each state’s redistricting body.

All redistricting plans must comply with equal population and the Voting Rights Act. The PL 94-171 dataset is used for assuring equal population. In order to check if a map complies with the Voting Rights Act you may, among other steps, need to conduct a Racially Polarized Voting Analysis. The inclusion of community of Interest maps is a common redistricting criteria that is required for legally compliant district maps. And of course, states adopt other criteria for state legislative and congressional redistricting, which may require election results, boundary files, and other data.

What data do you need for redistricting?

States use the decennial census data, and specifically the PL 94-171 dataset, to ensure districts have equal population. States may require minimizing county or municipal splits, or preservation of the core of prior districts, which means that different geographic and political boundary lines may be required. To comply with the Voting Rights Act (VRA), states need data on voters and constituents, including their race or ethnicity, such as voter file data, Citizen Voting Age Population, or American Community Survey data. They also need precinct boundaries joined with election results, not only for the VRA, but also to meet criteria around partisan fairness and competitiveness, or assess maps for partisan gerrymandering. Similarly, population projections can be used to judge the potential impact of a redistricting plan over the next decade. And many states require consideration of Communities of Interest, data which we are currently in the process of collecting.

Why do we redistrict?

The US Constitution requires a census of the population and a reapportionment of congressional individuals every ten years. The loss or gain of additional congressional seats is one reason to redistrict.

Regular redistricting is also required in order to ensure compliance with the principle of “one person, one vote.” This principle requires congressional districts to be “as nearly equal in population as practicable,” and legislative and local districts to be “substantially equal” in population. This principle comes from a series of rulings by the US Supreme Court in the 1960s that required a majority of states to redistrict, after years of legislative inaction had led to gross malapportionment. Malapportionment occurs when representatives have unequally sized constituencies, meaning that these constituencies have unequal voting power in the legislature. If our population never changed, there would be no need to redistrict! But because populations change over time, it is necessary to redraw the lines and ensure equal representation in government.

It is also important to note that there are other reasons to redistrict. Perhaps the most important is the Voting Rights Act (VRA), and its prohibition against minority vote dilution. The VRA requires that minority groups have the opportunity to elect candidates of their choice. Thus, even if the size of the population never changed, redistricting could still be required due to changes in the demographics of communities over the last decade.

When does redistricting occur?

In general, legislative and congressional redistricting occurs in the year or so following the release of the decennial census data. The census data is typically released by March 31 in years ending in 1 (in 2021, it was not released until August 12).

Every state has their own timeline for redistricting. Some states have constitutional or statutory deadlines for passing their maps. States also have filing deadlines for declaring to be a candidate in the 2022 primaries, which acts as a deadline for redistricting. And states vary widely in their public input process, which can range from a matter of a few days to several months.

Some states prohibit redistricting from occurring more often than once a decade, while others are ambiguous or silent on the matter. There is also litigation that can occur throughout the decade, which orders new redistricting for one or more districts.

Who does the redistricting?

Most commonly, committees in the state legislature are responsible for legislative and congressional redistricting. Like all redistricting bodies, they will draft maps using the mapping tool of their choice. Their plans must be approved by the legislature as a whole, then typically signed by the governor.

A number of states have also adopted commissions, although these come in many forms, ranging from independent, citizen commissions, such as those found in California and Michigan, to small, politician commissions, such as those found in Arkansas and New Jersey. Some states have advisory commissions, who can propose but not pass maps.

Since each state ultimately decides who will be responsible for redistricting, there are many variations from these general rules. And it is important to remember that different bodies might be responsible for legislative and congressional redistricting. You can find out who is responsible for redistricting in your state from All About Redistricting or the Brennan Center.

Participation in redistricting is possible for everyone: individuals, organizations, and educators. There are established groups that are involved in redistricting and participate by advocating for different causes. Members of the public can also submit testimony, attend meetings, or draw and submit maps.

Do you have more questions?

Our help desk team can answer your questions about redistricting data and the redistricting process. Send a message and they will respond within one business day!