Article 1 Section 2 of the Constitution affords Congress the power to apportion representatives “among the several States…according to their respective numbers.” For the House of Representatives, representation is based on population, so a given state can gain or lose seats depending on its population growth rate. However, since the United States population as a whole as grown rapidly since its independence, what becomes important for representation is a state’s growth rate relative to the growth rates of other states. From 2000 to 2010, only one state – Michigan – actually lost population, but eight other states lost representation because they did not grow as fast as the rest of the country.

The population changes are measured by a national census. The census, which the Constitution requires be taken every ten years, is carried out by the United States Census Bureau. Upon completion, the census apportions the number of Congressional seats a state will have in the House of Representatives for the next decade. The actual drawing of the districts is left to the states, but is subject to federal regulations. Currently, every state has the district lines drawn by either the state legislature or a specific redistricting commission. The federal regulations stem mostly from the Supreme Court. As a result of the “one person, one vote” cases of the 1960s, the Supreme Court ruled that districts must be drawn with as close to equal populations as is “practicable.” The Court also expects that districts be contiguous and compact. The lines must not close off one part of a district form the rest of the district, meaning that it is possible for one to walk from any point in the district to another without temporarily leaving the district. Also, whenever possible, line-drawers must avoid awkward and sprawling geometric shapes for the districts.

Redistricting is also governed by the Voting Rights Act (VRA), passed by Congress in 1965, and subsequent amendments. The VRA requires that areas with a history of voting discrimination pre-clear any changes in voting procedure, and allows lawsuits to be filed in districts suspected of being designed with discrimination in mind. VRA Amendments of 1982 encourage the creation of minority-majority districts.