by Stephen Ansolabehere and Maxwell Palmer
Last week, the Texas State Senate passed redistricting plan C141. The plan adds four new seats, which were allocated to Texas based on the 2010 census. This memo presents results using standard methods for assessing the likely partisan consequences of plans. We map past election results into the current districts in order to calculate what percent of seats are majority Republican and what percent of seats are majority Democrat. We further consider the possibility of uniform shifts in the statewide vote, either in the direction of Democrats or Republicans, to assess the likely affects of the plan on the division of seats in future elections. Of particular interest is the projected division of seats in the hypothetical case when the statewide vote is divided evenly between the parties. This value is called the “partisan bias” of the districting plan (see Browning and King 1987).
Based on the 2008 presidential election results, twenty-two of the twenty-three current Republican members of Congress will be in districts in which Republicans are expected to receive 55 percent of the vote or more, and eight of the nine current Democratic members of Congress will be in districts in which Democrats are expected to receive 55 percent of the vote or more. Democrat Lloyd Doggett’s 25th district will go from 60% Democratic to 55% Republican. Of the four new districts, two are districts where we expect Republicans to receive 55% or the vote or more, and two are districts where we expect Democrats to receive 55% of the vote or more. Consequently, if Doggett is defeated due to the shift of his district from heavily Democratic to Republican, the Republicans will increase their congressional delegation from twenty-three to twenty six seats, and the Democrats from nine to ten seats.
Analysis of the plan also allows us to project the likely division of the legislature for different (hypothetical) divisions of the vote statewide. We plot these results on a seats-votes curve, where each point on the plot represents the percentage of seats that would be won by the Democrats for the given vote share.
This curve reveals two important features of this plan. First, the plan has a partisan bias of 14-17%. Rather than winning 50% of the seats in the hypothetical case where the Democrats win 50% of the vote, the Democrats would win only 43-46% of the seats. To win 50% of the seats, the Democrats would need to win roughly 52-53% of the vote.
Second, the number of seats won by each party is constant for any Democratic share of the vote between thirty-seven and forty-eight percent. This range includes the recent statewide performance of most Democratic candidates over the last ten years, which has averaged 42%. This flat portion of the seats-votes curve indicates extremely low competitiveness in almost all of the congressional districts under this plan. Thus, the plan is not responsive to small changes in the vote share of either party in the range of vote shares that we expect in the next elections. All of the changes in the makeup of Texas’ congressional delegation are likely to be the result of the partisan decisions in the redistricting process, rather than from competitive congressional elections.
 Root, Jay. Texas Senate Approves GOP-Drawn Congressional Map. The Texas Tribune, June 6, 2011. http://www.texastribune.org/texas-redistricting/redistricting/texas-sena...
 We generated three different uniform swing curves using different estimates for the Democratic share of the vote. First, we used a normal vote, which is the mean of the Democratic vote in each voting tabulation district (VTD) for all contested elections between 2002 and 2010. Second, we used the 2008 Presidential vote. Third, we used the 2010 gubernatorial vote.