Ada County leans slightly Republican by roughly 8 points: about 46% of voters vote Democratic and 54% Republican.
About 75% of adults in Ada County typically vote, above the U.S. average of about 62%. Among adults in Ada County, ~35% vote Democratic, ~40% Republican, and ~25% don't vote. The map below shows estimated turnout by block group.
How Ada County compares
Among counties within 50 miles, Ada County is the least Republican-leaning.
Ada County runs about 28 points more Democratic than Idaho as a whole.
Politics vary noticeably by city within Ada County. The east side runs the most Democratic (D+33) and the southwest side runs the most Republican (R+38), a spread of about 71 points.
Why Ada County leans the way it does
This analysis examined 14,881 data points per county to find what predicts political lean and turnout. The items below are a few correlations that stood out for Ada County, not a ranked or complete list of what matters most.
Ada County votes Republican even though it is densely developed (about 86%, far above the Idaho average of 18%). State and regional patterns outweigh the Democratic lean that density usually predicts here.
Walkability and Democratic lean
Places with a highly walkable street grid tend to lean Democratic; Ada County, ID sits in the top tenth nationally on this measure. A walkable street grid does not change how people vote; it mostly reflects how urban a place is.
Why turnout in Ada County looks the way it does
Areas with strong routine healthcare access turn out at higher rates. Ada County is in the top quarter nationally for routine-care measures such as insurance coverage, preventive screenings, and dental visits. The dental-visit rate here is about 71%, about 11 points above the U.S. average of 60%. High high-school completion lines up with higher turnout, and about 95% of adults in Ada County have completed high school, above 92% of counties. Learn more about the findings and methodology on the political spectrum map.
Nearby Counties
- Canyon County, ID R+40
- Gem County, ID R+58
- Owyhee County, ID R+63
- Boise County, ID R+50
- Payette County, ID R+57
- Malheur County, OR R+42
- Elmore County, ID R+42
- Washington County, ID R+57
- Camas County, ID R+72
- Valley County, ID R+41
Counties with Similar Populations
- Richmond County, NY R+21
- Polk County, IA D+12
- Lake County, IN D+14
- Sonoma County, CA D+42
- Clark County, WA D+7
- Washoe County, NV D+3
- Prince William County, VA D+21
- Morris County, NJ Even
- Knox County, TN R+13
- Onondaga County, NY D+21
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Sources and methodology
Precinct-level voting records used to fit the model come from Idaho Secretary of State, Elections, distributed by the Voting and Election Science Team. Demographic inputs come from the U.S. Census Bureau (ACS 5-year estimates and the 2020 Decennial Census). Health and environmental inputs come from the CDC (PLACES and the Environmental Justice Index). Land cover comes from the USGS and EPA. Election-day and lead-up weather come from PRISM 4km daily grids and the NOAA Global Historical Climatology Network. Mail-voting and election-administration patterns come from the MIT Election Lab's Survey of the Performance of American Elections. Block-group crime detail comes from CrimeGrade. Internet data and modeling support provided by ISPreports.org.
Modeling and analysis by the BestNeighborhood data science team. Full methodology and findings: political spectrum map.
Methodology reviewed by the BestNeighborhood data team. Last updated May 2026.