The method to create congressional districts varies by state. The population according to the decennial U.S. census determines the number of voters in each congressional district. After states receive population and other demographic information, each state determines the boundaries of the district.
Most states require legislative approval for congressional boundaries, as of 2015. Some states also employ gubernatorial veto power, approval from state supreme courts or the use of a politician commission, a partisan commission that approves draws and approves congressional boundaries. A smaller number of states use advisory or independent committees to devise district boundaries and submit proposals to the state legislature for approval. For example, Iowa uses a nonpartisan committee, comprising state employees, and a five-person committee appointed by the majority and minority leaders in the legislature to devise boundary proposals. Arizona, Idaho, California and Washington all use single independent commissions.
Each state decides the criteria for drawing district boundaries. Some states attempt to account for certain communities while creating districts, such as the population of certain races or the mix of urban and rural residents. Gerrymandering is when a state adopts congressional districts, or any electoral district, that purposefully favors one political party or politician over another.