By Andy Jackson and Leah Byers
- The Civitas Partisan Index is a measure of the partisan leanings of North Carolina state legislative districts
- Court-ordered redistricting has changed the CPI of many NC House and Senate districts
- While the CPI is not a tool for predicting elections, readers can use it to learn more about state legislative races this year
With most of the primaries behind us, this is a good time to get a lay of the electoral landscape ahead of the general elections. With that in mind, we are releasing the 2020 Civitas Partisan Index (CPI).
How we measure the political leanings of NC legislative districts
The CPI is modeled after the Cook Political Report’s Partisan Voting Index. It is a measure of the base partisan leanings of a North Carolina state legislative district compared to the state as a whole. CPI is the party (D or R) followed by a number indicating the relative partisan lean of that district. For example, a district that tends to vote about 12 percent more Democratic than the state average would have a CPI score of D+12.
The 2020 CPI took precinct-level results of the ten 2016 Council of State races and the four 2018 statewide judicial races. The Republican votes of 14 races were added together in each state legislative district and divided by the total two-party votes for those 14 races to get a Republican percentage in each district. That percentage is then subtracted by the Republican statewide average for those 14 races (51.2%) to find the CPI rating for each district. By considering the deviation from the statewide average, the CPI somewhat mitigates effects such as incumbency, uneven races, or “wave” elections.
The 2016 Council of State races used in the CPI were for governor, lieutenant governor, commissioner of agriculture, commissioner of insurance, commissioner of labor, attorney general, secretary of state, superintendent of public instruction, state treasurer and state auditor. The 2018 statewide judicial races were for one seat on the North Carolina Supreme Court and three seats on the North Carolina Court of Appeals.
Changes from the 2018 CPI to the 2020 CPI
There are two ways in which the 2020 CPI differs from the 2018 version. The first is that we have included data from four 2018 statewide judicial races (one for the NC Supreme Court and three for the NC Court of Appeals) in order to better account for shifts within the North Carolina electorate. Doing so caused minor shifts with some rural districts (especially in the east) moving toward the Republicans by a percentage point or two and some urban districts moving toward the Democrats by a point or two. (We also included the governor’s race in the CPI for the first time, but the impact of that inclusion was minimal).
However, a much bigger change came from court-ordered redistricting. That redistricting shifted the CPI rating of numerous districts in both the NC House and NC Senate. Some of the races most affected by redistricting are listed in the table below:
We have also changed how we designate districts with close CPI ratings. We had previously labeled races with a CPI rating of two, one or zero as “swing Democrat” or “Swing Republican.” For the 2020 CPI we designate races with a CPI rating of one or zero as “toss up” and have shifted districts with a CPI rating of two in either direction into the “lean Democrat” or “lean Republican” categories. We did that to present our data in a format with which most readers would be familiar. Table one above reflects the new labeling.
How to use the 2020 CPI
The CPI is not a tool for predicting races; to do that you would need to include measures such as incumbency, coattail effects derived from presidential polling, and some measure of candidate quality such as funds raised. However, someone using the CPI without any other data would have successfully predicted 90 percent of state legislative races in 2018.
What the CPI does do is measure the general tendency of voters in the district to vote for one party or the other. That creates a baseline measure that journalists, researchers, and others can use when examining NC legislative races. By using CPI, you can get important insights into the 2020 election.