San Mateo leans heavily Democratic by roughly 48 points: about 74% of voters vote Democratic and 26% Republican.
About 60% of adults in San Mateo typically vote, near the U.S. average of about 62%. Among adults in San Mateo, ~44% vote Democratic, ~16% Republican, and ~40% don't vote. The map below shows estimated turnout by block group.
How San Mateo compares
Among cities within 25 miles, San Mateo leans more Democratic than 37 of 67 neighbors.
San Mateo runs about 29 points more Democratic than California as a whole.
Why San Mateo leans the way it does
This analysis examined 14,881 data points per city to find what predicts political lean and turnout. The items below are a few correlations that stood out for San Mateo, not a ranked or complete list of what matters most.
Dense areas vote Democratic. About 96% of residents in San Mateo live in densely developed areas, about 59 points above the U.S. average of 36%. High college attainment predicts Democratic voting, and San Mateo sits in the top quarter (about 57%, above 96% of cities). A high never-married share predicts Democratic voting, and about 35% of adults in San Mateo have never been married, above 88% of cities.
Population density and Democratic lean
Places with high population density tend to lean Democratic; San Mateo, CA sits in the top tenth nationally on this measure.
Why turnout in San Mateo looks the way it does
Renters vote less often than owners. About 48% of households in San Mateo rent, about 23 points above the U.S. average of 25%. Crowded housing lines up with lower turnout, and about 8% of homes in San Mateo have more than one occupant per room, above 95% of cities. Learn more about the findings and methodology on the political spectrum map.
Nearby Cities
- Hillsborough, CA D+41
- Foster City, CA D+50
- Highlands-Baywood Park, CA D+52
- Belmont, CA D+53
- Burlingame, CA D+52
- San Carlos, CA D+57
- Millbrae, CA D+40
- Redwood City, CA D+53
- North Fair Oaks, CA D+49
- San Bruno, CA D+43
Cities with Similar Populations
- San Angelo, TX R+36
- Decatur, GA D+74
- Kenosha, WI D+14
- Arden-Arcade, CA D+25
- League City, TX R+21
- Inglewood, CA D+60
- Sandy Springs, GA D+27
- Vacaville, CA D+5
- Frederick, MD D+27
- Stone Mountain, GA D+67
Sources and methodology
Precinct-level voting records used to fit the model come from California 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.