To identify the most Democratic-leaning county in each state, 24/7 Wall St. created an index from various political characteristics of U.S. counties. The index comprises three subindices: county representation in the U.S. House of Representatives for the last five congressional cycles, state representation in the U.S. Senate for the last five congressional cycles, and the share of votes cast in the 2012 presidential election. Each subindex ranges from -1 to 1, with extreme values given to counties where representation or voting shares were dominated by a single party, either Republican or Democrat. The bluest county in each state is the highest sum of our three indices. Our analysis included only counties that cast more than 2,000 total votes. The percentage of county residents who had attained at least a bachelor’s degree, median household income, poverty rates, and the share of residents identifying as white come from the U.S. Census bureau’s 2015 American Community Survey five-year estimates.