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Redistricting is the process of drawing United States electoral district boundaries.
In 32 states, the state legislature has primary responsibility for creating a redistricting plan, in many cases subject to approval by the state governor. To reduce the role that legislative politics might play, seven states (Arizona, California, Hawaii, Idaho, Montana, New Jersey and Washington) determine congressional redistricting by an independent or bipartisan redistricting commission. Four states, Florida, Iowa, Maine, and New York give independent bodies authority to propose redistricting plans, but preserve the role of legislatures to approve them. Seven states have only a single representative for the entire state because of their low populations; these are Alaska, Delaware, Montana, North Dakota, South Dakota, Vermont and Wyoming.
State constitutions and laws also mandate which body has responsibility over drawing the state legislature boundaries. In addition, those municipal governments that are elected on a district basis (as opposed to at-large) also redistrict.
Each state has its own standards for creating Congressional and legislative districts. In addition to equalizing the population of districts and complying with Federal requirements, criteria may include attempting to create compact, contiguous districts, trying to keep political units and communities within a single district, and avoiding the drawing of boundaries for purposes of partisan advantage or incumbent protection. In the states where the legislature (or another body where a partisan majority is possible) is in charge of redistricting, the possibility of gerrymandering (the deliberate manipulation of political boundaries for electoral advantage, usually of incumbents or a specific political party) often makes the process very politically contentious, especially when the majorities of the two houses of the legislature, or the legislature and the governor, are from different parties. The state and federal court systems are often involved in resolving disputes over Congressional and legislative redistricting when gridlock prevents redistricting in a timely manner. In addition, the losers to an adopted redistricting plan often challenge it in state and federal courts. Justice Department approval (which is known as pre clearance) was formerly required under Section 5 of the Voting Rights Act of 1965 in certain states that have had a history of racial barriers to voting.
Partisan domination of state legislatures and improved technology to design contiguous districts that pack opponents into as few districts as possible have led to district maps which are skewed towards one party. Consequently, many states including Florida, Georgia, Maryland, Michigan, Pennsylvania and Texas have succeeded in reducing or effectively eliminating competition for most House seats in those states. In 2003 Republicans in Texas increased their representation in the U.S. House through a controversial mid-decade redistricting.
Other states including California, New Jersey and New York have opted to protect incumbents of both parties, again reducing the number of competitive districts. The Supreme Court's ruling on the Pennsylvania gerrymander effectively cemented the right of elected officials to select their constituents by eliminating most of the grounds for disenfranchised constituents to challenge gerrymandered lines.
Redistricting has to follow certain criteria to be accepted.
- equal population
- preservation of existing political communities
- partisan fairness
- racial fairness 
Redistricting is also necessary within (but not among) school districts, where attendance zones have grown (or occasionally shrunk) disproportionately to the occupancy capacity of each public school in the system. This always occurs when a new school is built or one is closed, but may also occur due to other shifts in population. These districts are necessary not only to balance enrollment, but also to coordinate school bus routes. Separate maps are usually kept for each level: elementary school, middle school, and high school, for example. This is not an inherently political process, however parents can become very upset when their children are moved from a school they like (or to one they don't), and occasionally elected school boards have been forced to change plans after protests. Even those without school-age children may take an interest in school redistricting, as it is perceived to affect the resale value of a home, and real estate agents and companies (and "for sale by owner" listings) often list school attendance districts for each home as a selling point.
Gerrymandering is the practice of drawing district lines to achieve political gain for legislators. The practice of gerrymandering involves the manipulation of district drawing in aims to leave out, or include, specific populations in a legislator's district to ensure his/her reelection.
- "2009 Redistricting Commission Table". National Conference of State Legislatures (NCSL). June 28, 2008. Retrieved 2013-09-06.
- Blake, Aaron. "Government Redistricting Web Sites". Purdue University Libraries. Retrieved 2009-08-25.
- "TheHill.com - Redistricting looms over 2010 landscape". thehill.com. Retrieved 2009-08-25.
- Miller, Jason C.,Community as a Redistricting Principle: Consulting Media Markets in Drawing District Lines (July 6, 2010). Indiana Law Journal Supplement, Vol. 5, 2010.
- "Vieth v. Jubelirer". supct.law.cornell.edu. Retrieved 2009-08-25.
- Jacobson, Gary (2013). The Politics of Congressional Elections. New Jersey: PEARSON Education. p. 9.
- All About Redistricting Includes state criteria.
- Creating Fair Congressional Districts A site with an algorithm using just population and state shape for creating congressional districts.
- redistrictinginamerica.org A comprehensive source for information about redistricting in all fifty states from the Rose Institute of State and Local Government
- MappingSoftware.com Maptitude for Redistricting News
- Congressional Apportionment from the Office of the Clerk at the United States House of Representatives, including historical representation by state
- District sizes and other data from 1900-2000 from the United States Census
- Government Redistricting Web Sites from GovDocs at Purdue University Libraries, includes list of state websites
- Public Mapping Project
- A Citizen's Guide to Redistricting, 2010 Edition, downloadable
- Reapportionment and Redistricting in the U.S. from the ACE Project
- Rodriguez, Lori. "Getting point of redistricting." Houston Chronicle. Saturday August 24, 1991. A25.
- Jeffrey B. Lewis; et al. (2013). "Digital Boundary Definitions of United States Congressional Districts, 1789-2012". University of California, Los Angeles.
- "Redistricting: How Powerful Interests Are Drawing You Out of a Vote". ProPublica.
- The Redistricting Game - Where Do You Draw the Lines A simulation of how redistricting works. It uses the real US laws and practices and incorporates quotes from US political leaders.