2024
D+50.3marginKamala HarrisDem
242,957
73.5%
Donald TrumpRep
76,616
23.2%
All other candidates
10,937
3.3%
330,510 ballots cast · 44.6 per 100 residents (2024 population estimate)
Residents740,627−2.9% since 2020 (−22,012 people)
San Mateo County
California·FIPS 06081·743,568 residents (2025 estimate)
2024 result
D+50.3
330,510 ballots cast
What happened: the county's margin each cycle, against the national result.
Lean vs. nation
D+53.6
How this county sits relative to the country in 2024, with the national environment removed.
Trend since 2000
+19.2 pts
Net movement of that lean across 7 cycles — toward Democrats.
Elasticity
1.00
Moves roughly in step with the country.
Bellwether error
47.2
Average gap between this county's margin and the nation's.
Votes by candidate
7 cyclesCertified vote counts for each party's nominee, the combined total for everyone else on the ballot, the number of ballots cast, and how many people lived here at the time.
2020
D+57.7marginJoe BidenDem
291,496
77.9%
Donald TrumpRep
75,584
20.2%
All other candidates
7,171
1.9%
374,251 ballots cast · 49.1 per 100 residents (2020 population estimate)
Residents762,639−0.7% since 2016 (−5,267 people)
Measured across a change of Census vintage. The two series share no common year, so how far apart they sit cannot be measured — this change carries an unknown amount of revision on top of four years of real change.
2016
D+57.2marginHillary ClintonDem
237,882
75.7%
Donald TrumpRep
57,929
18.4%
All other candidates
18,573
5.9%
314,384 ballots cast · 40.9 per 100 residents (2016 population estimate)
Residents767,906+3.9% since 2012 (+28,682 people)
2012
D+46.7marginBarack ObamaDem
206,085
72.1%
Mitt RomneyRep
72,756
25.5%
All other candidates
6,879
2.4%
285,720 ballots cast · 38.7 per 100 residents (2012 population estimate)
Residents739,224+5.0% since 2008 (+35,394 people)
Measured across a change of Census vintage. Where the two vintages overlap, 96% of counties agree within 0.5%, so read this figure to about half a point and no finer.
2008
D+48.7marginBarack ObamaDem
222,826
73.5%
John McCainRep
75,057
24.7%
All other candidates
5,409
1.8%
303,292 ballots cast · 43.1 per 100 residents (2008 population estimate)
Residents703,830+2.0% since 2004 (+13,669 people)
2004
D+40.3marginJohn KerryDem
197,922
69.7%
George W. BushRep
83,315
29.3%
All other candidates
2,840
1.0%
284,077 ballots cast · 41.2 per 100 residents (2004 population estimate)
Residents690,161−2.5% since 2000 (−17,659 people)
2000
D+33.3marginAl GoreDem
166,757
64.3%
George W. BushRep
80,296
31.0%
All other candidates
12,346
4.8%
259,399 ballots cast · 36.6 per 100 residents (2000 population estimate)
Residents707,820
“All other candidates” is every remaining line on the presidential ballot combined — minor parties, independents and write-ins where the state reports them. Shares are of all ballots cast, so the three rows sum to 100%.
Ballots cast per 100 residents is not a turnout rate. It measures votes against every resident — children and non-citizens included — not against eligible or registered voters, which is what turnout means and which this site does not have a county-level figure for. It is shown only where an election year and a population estimate share the same year and describe the same territory; where the Census figure is stated on boundaries the vote was not cast on, the card says so and the ratio is withheld.
A change in residents is not a change in voters. The percentage beside each year is the change in resident population since the previous presidential election — everyone who lives here, including children and non-citizens. It is not turnout, not registration, and not the same quantity as the change in ballots cast. Population is measured on 1 July of the election year, four months before the vote, which is the closest an annual series gets to election day.
Population figures on this page come from 3 Census series: PEP intercensal county estimates, 2000–2010; PEP vintage 2019 county estimates, 2010–2019; PEP vintage 2025 county estimates, 2020–2025. The 2012 and 2020 figures are the first in a new series, so the change shown for those years is measured across a change of Census method as well as across four years. Each of those cards says so. Two figures drawn from different vintages are not two readings of one series.
Every cycle
| Year | Margin | Lean | Swing | Rel. swing | Votes | 3rd party |
|---|---|---|---|---|---|---|
| 2024 | D+50.3 | D+53.6 | -6.8 | -0.7 | 330,510 | 3.3% |
| 2020 | D+57.7 | D+54.3 | -2.0 | -4.3 | 374,251 | 1.9% |
| 2016 | D+57.2 | D+58.6 | +13.0 | +14.8 | 314,384 | 5.9% |
| 2012 | D+46.7 | D+43.8 | -1.8 | +1.6 | 285,720 | 2.4% |
| 2008 | D+48.7 | D+42.2 | +8.9 | -1.0 | 303,292 | 1.8% |
| 2004 | D+40.3 | D+43.2 | +5.8 | +8.9 | 284,077 | 1.0% |
| 2000 | D+33.3 | D+34.4 | — | — | 259,399 | 4.8% |
Margin is over all votes cast. Lean is the two-party margin minus the nation's — the county's own position with the national environment removed. Relative swing is the change in that lean. “Votes” is the number of ballots cast in the presidential contest, not a share of eligible voters.
Counties on a similar trajectory
Sources
Presidential returns from the MIT Election Data and Science Lab (County Presidential Returns 2000–2024, CC0). Geography and FIPS codes from the US Census Bureau (public domain). Where a state reports both a total and per-mode vote counts, the reported total is used.
Resident population from the US Census Bureau, Population Estimates Program (Vintage 2025), measured as of July 1, 2025. Resident population — all residents, including children and non-citizens. Not an electorate, and not comparable to a vote total as a turnout rate. Conclusions drawn from it are ours, not the Bureau's.
Population at each election is measured on 1 July of the election year and comes from the US Census Bureau's Population Estimates Program (public domain). The 2000–2024 span is covered by several separately estimated vintages of that programme rather than by one continuous series — the ones used on this page are named under the cycle cards, and every change measured across a boundary between them is marked there. Notes about territory changing hands come from this site's own change ledger and are only as complete as it is.