San Mateo County, CA Political Map | Democrat & Republican Areas in San Mateo County

By the BestNeighborhood data science team. Last updated May 2026.

San Mateo County leans heavily Democratic by roughly 48 points: about 74% of voters vote Democratic and 26% Republican.

 

About 61% of adults in San Mateo County typically vote, near the U.S. average of about 62%. Among adults in San Mateo County, ~45% vote Democratic, ~16% Republican, and ~39% don't vote. The map below shows estimated turnout by block group.

How San Mateo County compares

Among counties within 50 miles, San Mateo County leans more Democratic than 3 of 7 neighbors.

San Mateo County runs about 27 points more Democratic than California as a whole.

Politics vary noticeably by city within San Mateo County. The southeast side is the most Democratic-leaning (D+55) and the southwest side is the least Democratic-leaning (D+40), a spread of about 15 points.

Why San Mateo 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 San Mateo County, not a ranked or complete list of what matters most.

Dense areas vote Democratic. About 90% of residents in San Mateo County live in densely developed areas, about 54 points above the U.S. average of 36%. High college attainment predicts Democratic voting, and San Mateo County sits in the top quarter (about 54%, above 98% of counties). A high never-married share predicts Democratic voting, and about 34% of adults in San Mateo County have never been married, above 81% of counties.

Walkability and Democratic lean

Places with a highly walkable street grid tend to lean Democratic; San Mateo County, CA 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 San Mateo County looks the way it does

Renters vote less often than owners. About 40% of households in San Mateo County rent, about 15 points above the U.S. average of 25%. Strong routine healthcare access lines up with higher turnout, and San Mateo County sits in the top quarter on routine-care measures. Crowded housing lines up with lower turnout, and about 8% of homes in San Mateo County have more than one occupant per room, above 97% of counties. Learn more about the findings and methodology on the political spectrum map.

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.