Because population data does not provide information about the political composition of a proposed district, political data is sometimes added to a redistricting database. Political data may include statistics on the partisan affiliation of electors (if available) and tabulations of votes for candidates and ballot measures in prior elections. When political information is included in a redistricting database, it may be possible to predict how support for a particular candidate or political party might be affected by a change in the geographic composition of a district.
The major consumers of political information in the redistricting process are political parties and incumbent legislators. Political parties are interested in determining how changes in district boundaries are likely to affect the election of their candidates and, ultimately, the partisan composition of the legislature. Incumbent legislators, of course, are interested in ensuring that they will continue to be re-elected by the voters in their new districts.
In the United States, the courts are also interested in political information, but usually for very different reasons. Under the Voting Rights Act of 1965, certain minority groups are protected from redistricting plans that might dilute the effect of their vote. The courts use election results from prior contests in minority vote dilution claims to determine if there is (1) minority group cohesion in support of certain candidates and (2) bloc voting on the part of white voters against the minority group's preferred candidates. If these two conditions are met, and if the minority group is sufficiently large and geographically compact, then boundaries must be redrawn to create a district in which the minority community has the opportunity to elect a representative of its choice. (For more information on minority vote dilution cases in the United States, see Role of the Courts in Electoral District Delimitation.)
Problems with Using Political Data for Redistricting
Adding political data to a redistricting database may be problematic since election geography may not correspond with census geography. In the United States, for example, where census geography is usually used to redraw districts, the Census Bureau requires that census geographic units--census blocks and tracts--follow easily recognised features, such as roads and rivers. The boundaries of election precincts, however, have historically been based on property lines and therefore do not necessarily coincide with the boundaries of census geographic units.
If political data is added to a redistricting database in the United States, the data must somehow be matched to its corresponding census geography. This process can be difficult and time-consuming, as well as fraught with inaccuracies. Countries that redistrict on the basis of voter registration figures do not face this problem since registration figures and political data are usually reported at the same geographic level.
Advantages and Disadvantages of Political Data
The primary disadvantage associated with the use of political data is that it enables redistricters to create a districting plan that may favour one political party at the expense of others. In countries that have neutral boundary commissions, political information is often purposely excluded from redistricting databases to prevent political gerrymandering. Of course, political parties with sufficient resources may collect their own political data to learn in advance about the potential partisan implications of a redistricting plan.
The advantage of including political information in a redistricting database is that the partisan implications of a new redistricting plan may be determined prior to its implementation. And the ability to make such a determination is not limited solely to political parties or special interest groups with sufficient funds to carry out the necessary analysis. Since any changes in boundary lines are likely to have partisan consequences, whether intentional or not, it may be better to know in advance what these effects will be.
Of course, the partisan impact of a redistricting plan is not necessarily predictable, especially if voters are not strongly partisan. It is an inexact science in any case, and some rather infamous gerrymandering has backfired on their creators.
