Where are the lines drawn?
- Redistricting Criteria
Those who have the redistricting pen don't have a blank slate for drawing the lines. Various rules and constraints limit where district lines may or may not be drawn. For additional resources on these criteria, see here.
- State criteria
Even after accounting for the federal rules, there are countless ways to divide a jurisdiction into districts. State constitutions -- and sometimes, state statutes -- provide a few additional rules for drawing the lines.
Occasionally, states list these constraints in priority order, and expressly require satifying high-priority criteria before turning to the others. More often, though, the principles below are simply presented without express priorities, and with instructions to follow them "where practicable." When that happens, the entity drawing the lines has the discretion to take criteria into account, and resolve conflicts where they arise, as it sees fit.
Contiguity is the most common rule imposed by the states: by state constitution, statute, or guideline, 49 states require at least one chamber's state legislative districts to be contiguous. 23 states have similarly declared that their congressional districts will be contiguous. (The smaller number reflects the fact that few states have any express legal constraints on congressional districting. In practice, the vast majority of congressional districts -- perhaps every one in the 2010 cycle -- will be drawn to be contiguous.)
A district is contiguous if you can travel from any point in the district to any other point in the district without crossing the district's boundary. Put differently, all portions of the district are physically adjacent. Most states require portions of a district to be connected by more than a single point, but don't further require that a district be connected by territory of a certain area.
Few redistricting concepts are absolute, and contiguity is no exception. Many states require contiguity only "to the extent possible," and courts generally accept anomalies that otherwise seem reasonable in context. For example, the city of Racine, Wisconsin, has a non-contiguous boundary (boundaries like this are fairly common by-products of annexation). And so the legislature drew Wisconsin's 61st state assembly district to incorporate most of the city of Racine -- with a noncontiguous portion of the district embracing the noncontiguous portion of the city.
Water also gets special treatment for contiguity. In most cases, districts divided by water are contiguous if a common means of transport (like a bridge or ferry route) connects the two sides of the district. Island districts are generally contiguous as long as the island is part of the same district as the mainland area closest to the island or most tied to the island by these sorts of transport routes. In Hawaii, where there is no mainland to consider, the state constitution prohibits the drawing of "canoe districts" -- districts that are spread across more than one major island group, where it is necessary to use a "canoe" to travel between different parts of the district.
- Political boundaries
The next most common state rule is a requirement to follow political boundaries, like county, city, town, or ward lines, when drawing districts. By state constitution, statute, or guideline, 42 states require state legislative districts to show some accounting for political boundaries; 19 states impose similar constraints on congressional districts. Most often, state law concerning political boundaries leaves a fair amount of flexibility in the mandate -- one common instruction is to keep to political boundaries "to the extent practicable." And like all other state redistricting law, this rule must bend where necessary to federal equal population or Voting Rights Act constraints.
It is worth remembering that some cities or towns spill over county lines; even though counties are usually bigger than cities, keeping strictly to county lines may mean cutting off pieces of these "spillover" cities or metropolitan areas.
Also, if counties or cities have to be split to comply with other redistricting requirements, most state law does not specify whether it is better to minimize the number of jurisdictions that are split, or to minimize the number of times that a jurisdiction is split. The former might mean splitting a few jurisdictions into many pieces; the latter might mean splitting a greater number of jurisdictions, but into fewer pieces.
Almost as often as state law asks districts to follow political boundaries, it asks that districts be "compact." 37 states require their legislative districts to be reasonably compact; 18 states require congressional districts to be compact as well.
Few states define precisely what "compactness" means, but a district in which people generally live near each other is usually more compact than one in which they do not. Most observers look to measures of a district's geometric shape. In California, districts are compact when they do not bypass nearby population for people farther away. In the Voting Rights Act context, the Supreme Court seems to have construed compactness to indicate that residents have some sort of cultural cohesion in common.
Scholars have proposed more than 30 measures of compactness, each of which can be applied in different ways to individual districts or to a plan as a whole. These generally fit into three categories. In the first category, contorted boundaries are most important: a district with smoother boundaries will be more compact, and one with more squiggly boundaries will be less compact. In the second category, the degree to which the district spreads from a central core (called "dispersion") is most important: a district with few pieces sticking out from the center will be more compact, and one with pieces sticking out farther from the district's center will be less compact. In the third category, the relationship of housing patterns to the district's boundaries is most important: district tendrils, for example, are less meaningful in sparsely populated areas but more meaningful where the population is densely packed.
In practice, compactness tends to be in the eye of the beholder. Idaho, for example, says that its redistricting commission "should avoid drawing districts that are oddly shaped" -- which is more specific than most states. Only 6 states appear to specify a particular measure of compactness: Arizona and Colorado focus on contorted boundaries; California, Michigan, and Montana focus on dispersion; and Iowa embraces both.
- Communities of interest
Preserving "communities of interest" is the next most common criterion reflected in state law. 24 states consider keeping "communities of interest" whole when drawing state legislative districts; 13 states do the same for congressional districts.
A "community of interest" is just a group of people with a common interest (usually, a common interest that legislation might benefit). Kansas' 2002 guidelines offered a fairly typical definition: "[s]ocial, cultural, racial, ethnic, and economic interests common to the population of the area, which are probable subjects of legislation." Alabama adds the helpful reminder that "[i]t is inevitable that some interests will be recognized and others will not, [but] the legislature will attempt to accommodate those felt most strongly by the people in each specific location."
Several of the other principles above may be seen as proxies for recognizing rough communities of interest. For example, a requirement to follow county boundaries may be based on an assumption that citizens within a county share some common interests relevant to legislative representation. Similarly, a compactness requirement may be based on a similar assumption that people who live close to each other have shared legislative ends. But each of these proxies may also be imperfect: people with common interests don't generally look to geometric shapes -- or even strict political lines -- when they consider where they want to live. Considering communities of interest directly is a way to step past the proxy.
- Political outcomes
Most scholarly and popular attention to redistricting has to do with the political outcome of the process. But the law has relatively little to say about the issue.
The federal constitution, for example, puts few practical limits on redistricting bodies. Individual districts can be drawn to favor or disfavor candidates of a certain party, or individual incumbents or challengers (indeed, the Court has explicitly blessed lines drawn to protect incumbents). As for the district plan as a whole, all of the Supreme Court Justices apparently agree that excessive partisanship in the process is unconstitutional, but there is a vigorous dispute about the courts should decide how much is "too much," and if so, what the measure should be. In practice, the Court has never found such a case -- in fact, it has never agreed on a standard.
In most states, there are similarly few restrictions on political outcomes. Eight states prohibit their redistricting bodies, most of which are commissions, from drawing state legislative districts in order to "unduly" favor a candidate or political party; the same eight states do the same for congressional districts. Arizona, California, Iowa, Idaho, and Montana ban considering an incumbent's home address when drawing district lines; many of the same states also limit the use of further political data like partisan registration or voting history. Note: where minority populations present the possibility of obligations under the Voting Rights Act, those drawing the lines may have to consider partisan voter history to assess racial polarization, no matter what state law provides. Also, it is important to remember that every decision to draw district lines in one place or another has a political effect; lines drawn without looking at underlying voting data can be just as politically skewed as lines drawn with the data in mind.
Arizona and Washington, both states where independent commissions draw the lines, are the only states that affirmatively encourage districts that are competitive in a general election -- about the same number of Democrats and Republicans. In both states, this is a goal to be implemented only when practicable, and only when doing so would not detract from other state priorities.
- Other state rules
There are three other notable structural rules that, in some statse, govern the location of district lines. The first is a "nesting" requirement. In states where districts are "nested," the districts of the state Senate are constructed by combining two or three state House or Assembly districts (or the districts of the state House or Assembly are constructed by dividing up each state Senate district). In contrast, without nesting, the districts of each legislative house are independent; they may follow the same boundary lines, but they don't have to. In 14 states, the lower and upper legislative house districts are nested.
The second rule concerns districts where 2, 3, or more representatives are elected from the same district; these are called "multi-member" districts. Since 1842, federal law has prohibited multi-member districts for Congress, but many local legislatures still elect several representatives from a single district. In the state legislature, Arizona, New Jersey, South Dakota, and Washington elect all lower house members from multi-member districts; 10 other states expressly authorize the use of one or more multi-member districts; and five states have no law that either prohibits or expressly permits multi-member districts. In some instances, multi-member districts may be used together with nesting rules; in Arizona, for example, each district elects one state senator and two state representatives. In other cases, multi-member districts for one legislative chamber are not tied to the districts of the other chamber: a Senate district and a multi-member Assembly district are entirely unrelated. Multi-member districts in which each representative is elected by majority vote may raise concerns under the Voting Rights Act, though such concerns can be alleviated through some alternative voting rules.
The third rule of note is the "floterial" district: a district that wholly or partially overlaps other districts in the same legislative chamber. Florida, Mississippi, and New Hampshire expressly permit floterial districts; 32 states prohibit floterial districts; and the 15 other states appear to have no law regulating floterial districts one way or the other. Most floterial districts arose as a way to preserve political boundaries while also limiting severe population disparities. Imagine a state where the average district's population is 100, but there are two adjacent towns with 150 people each. One way to ensure equal population is to split up the towns so that there are three mutually exclusive districts with 100 people each. An alternative is to create one district serving each town, and one "floterial district" elected by the 300 people in both towns together, so that the 300 people have the same 3 total representatives.