In the 1960s the U.S. Supreme Court ruled in a series of cases that congressional and state legislative districts must be of equal population. The watershed case was Baker v Carr, where the Supreme Court ruled that redistricting was justicable, meaning that they could apply impose constitutional and statutory requirements upon redistricting. Prior to this ruling, the Supreme Court was reluctant to become involved in what they considered to be a political question best resolved by the political process, not the courts.
At stake was equal representation based on population. Some states had not redistricted in several decades, which created an population imbalance between districts located in fast growing urban areas and slow growing rural areas. This imbalance is called malapportionment, and led to unequal representation, with persons living in rural areas effectively having more representation in Congress and their state legislatures than those living in urban areas. With rural interests dominating state legislatures, the Supreme Court realized that the political process could not fix itself, and therefore ordered that districts must be of equal population. Many states read the writing on the wall and fixed their districts, and in some cases fixed their state constitutions to ensure redistricting would happen in a timely manner. Some did not, and the courts were forced to act. Among the most important of these cases are Wesberry v Sanders, which required congressional districts to be of equal population, and Reynolds v Sims, which required the same for state legislative districts.
Over time, the substance of these court rulings have become known by the phrase "one man, one vote." This phrasing is unfortunate not only because it is not politically correct, but because it is not what the court ruled. The court held the the 14th Amendment requires "one person, one unit of representation." That is to say that every person -- irregardless if they are unable to vote due to their citizenship status or voting age -- are afforded equal representation. Districts are therefore drawn on equal population, not on equal votes.
Some states modify their population counts for their state legislative redistricting. Hawaii and Kansas remove non-resident military and Kansas reallocates students to their homes.
Recently, Delaware, Maryland and New York passed laws that reallocate prisoners from their prisons to the communities whence they came. The issue at stake is that prison populations essentially constitute filler population since prisoners in most states cannot vote. The Census Bureau is making available group quarters data sometime in mid-2011 that will make it possible to adjust the census population counts for prisoners. More information on this issue can be found at Prisoners of the Census.
It may take some time between when the 2010 census data are released and when these states will adjust make these population adjustments. Districts that are drawn in these states using the 2010 census data may therefore be incorrect until these adjustments are released.
How Equal Is Equal?
For congressional districts, the Supreme Court ruled in Karcher v Daggett that congressional districts must have as equal population as possible, unless a state can provide a good reason why they must unbalance their congressional district populations. A recent ruling in Vieth v Jubelirer ruling suggests that congressional districts should have exactly the same population, if that is possible, or as much as a one person deviation among districts.
For state legislative districts, the Supreme Court has generally allowed greater leeway. The purpose in doing so was to allow states to respect traditional redistricting principles like drawing districts to follow county boundaries, as much as that is possible. In 1983, the Supreme Court ruled in Brown v Thomson that state legislative and local districts could vary by as much as a ten percent range.
The Supreme Court recently ruled in Cox v Larios that the supporters of a political party could not systematically be placed with large or small population districts within the 10% range, so that malapportionment could be used to affect the number of districts a party might be expected to win. Some have interpreted this to mean that for state or local districts to survive a court challenge these districts must also have as near equal population as possible, unless the state can show that there is a legitimate reason for unbalanced district populations, such as respecting a state constitutional requirement to respect county boundaries.
Some states may impose more strict population deviations than the ten percentage point range allowed by the U.S. Supreme Court.