Using simple measurements like incumbency, competitiveness (elections won by less than 75%), and previous election results since 2000, we predicted the likelihood of Democrats regaining control of the state legislature. In doing so, we also measured the current disadvantage Democrats face resulting from districting. In this post, we’ll discuss whether different redistricting schemes help reduce skewed proportionality regarding a legislature’s seat to vote distribution. To the left is an example of our predicted “vote to seat” distribution (this time in Iowa), which predicts the likely seats Democrats will receive given their state-wide vote total. If districts were ideally drawn, 50% of the votes statewide for Democrats would translate into 50% of the seats in the state assembly. Comparing three midwestern states (Iowa, Minnesota, and Wisconsin), we can see that different districting schemes yield different levels of proportionality. Since most elections are decided between 45%-55% of the vote, we can tell how volatile or unequal a system is by the steepness of the curve. A truly proportional system is represented by the dotted line in the graph to right, passing through 0,0 and 50,50. These trends are normal for majoritarian representation, but clearly some of these states are more proportional than others, so what causes the difference between these states?
There are many ways for states to redistrict, with some states allowing their legislatures to redraw districts exclusively, while others use independent commissions. Predicted mean values of the seat to vote distributions for IA, MN, and WI in 2012. A completely proportional system is represented by the dotted line. In our small sample, the state legislatures in Wisconsin and Minnesota have exclusive power over redistricting. Iowa, by comparison, has the Legislative Service Agency (LSA), which works with a temporary advisory commission to create a proposal and sends it to the state legislature to be approved.
Our hypothesis is that relatively non-partisan/commission led redistricting efforts like Iowa’s system results in lower levels of partisan bias (or disproportionality) since nonpartisan committees have less of an incentive to create a partisan-biased system. The result is one that should benefit voters, not parties. This line of reasoning is supported by a comparison of partisan bias between the states over the course of multiple censuses. Iowa fluctuates above and below zero. Iowa’s redistricting scheme allows for more variation in voter preference, and consequently party preference, whereas Democrats in both Minnesota and Wisconsin are at a clear disadvantage. Even though voters statewide in Wisconsin prefered Democrats (they received over 50% of the votes in 2012), Republicans disproportionality hold a 13% seat advantage. Although this graph is visually convincing, the claims are also statistically valid.