In the United States, voters choose representatives based on geographic districts. These districts determine how voters are grouped together when electing members of Congress or state legislatures. Because populations change over time, the boundaries of these districts must occasionally be adjusted. The process of redrawing those boundaries is called redistricting.
Redistricting has historically and usually occurred once every ten years after new population data becomes available from the Census. The goal is to ensure that districts represent roughly equal numbers of people. Population shifts occur as people move between regions, cities grow, or rural populations decline. If district boundaries stayed the same for long periods, some districts would represent far more people than others. Redistricting helps maintain the principle that each person's vote carries similar weight, especially in the House of Representatives.
Redistricting is related to another process called apportionment, which determines how many seats in the U.S. House of Representatives each state receives based on population. Once the number of seats assigned to a state is determined, that state divides its population into districts for those seats. States redraw district maps to reflect population changes and maintain equal representation.
Although federal law sets some requirements, much of the redistricting process is determined by state law. In many states, state legislatures draw district maps. In others, independent commissions or special boards take on that responsibility. Because district maps determine which voters are grouped together, the process can influence political representation and often receives public attention.
When drawing districts, mapmakers typically follow several widely used principles intended to support fair representation. These are often referred to as traditional districting criteria:
Equal population: Districts should contain roughly the same number of people so that each vote carries similar weight.
Contiguity: All parts of a district should be connected so that the district forms one continuous area.
Compactness: Districts should avoid unusually stretched or irregular shapes when possible.
Respect for political boundaries: District lines should try to follow existing county, city, or town boundaries.
Communities of interest: Districts may try to keep together groups of people who share common economic, cultural, geographic, or social interests.
These criteria are not always easy to satisfy at the same time. For example, keeping a community together might make a district less compact, while equal population requirements might require splitting a city across multiple districts. Mapmakers often balance several goals when drawing district boundaries.
District maps must also comply with federal legal standards. Congressional districts within a state must have nearly equal populations, a principle often called “one person, one vote.” Until recently, federal law was also understood to prohibit district maps that weakened or diluted the voting strength of racial or language minority groups. A recent Supreme Court ruling (Louisiana v. Callais) narrowed that understanding, making it harder to require race-conscious districting under the Voting Rights Act and limiting when states may rely on race in drawing districts.
To ensure that states follow these constitutional requirements, federal courts can review how district boundaries are drawn. In Baker v. Carr (1962), the Supreme Court ruled that citizens can also challenge unfair districting practices, helping ensure districts are updated to reflect population changes and provide equal representation. Special interest groups and advocacy organizations may also influence redistricting by supporting legal challenges or advocating for changes to the way district maps are drawn.
Despite these guidelines, redistricting can still be controversial. Because district boundaries influence which voters are grouped together, they can affect which candidates are more likely to win elections. When district lines are intentionally designed to advantage a political party, candidate, or group, the practice is commonly called gerrymandering.
Gerrymandering can occur in several ways. Mapmakers might concentrate voters with similar preferences into a single district so their influence is limited to one seat. In other cases, a group of voters might be divided across several districts so that they do not form a majority in any of them.
For these reasons, redistricting often leads to debate, court challenges, and political disagreement. Even with advanced mapping technology, determining how to balance population equality, representation, and other districting principles remains a complex process.
