Competition:  A district is considered competitive if the margin of victory in the two-party vote share is eight points or less.
Regions:  Midwest & Plains: IL, IN, IA, KS, MI, MN, MO, ND, NE, OH, SD, and WI.
Mountain States: AZ, CO, ID, MT, NV, NM, UT, and WY.
Northeast: CT, DE, ME, MD, MA, NH, NJ, NY, PA, RI, and VT.
South: AL, AR, FL, GA, KY, LA, MS, NC, OK, SC, TN, TX, VA, and WV.
West: AK, CA, HI, OR, and WA. 
Source: Brennan Center analysis based on Voting and Election Science Team (VEST) data, Harvard Dataverse.