San Diego County had historically been a Republican stronghold. The Republican presidential nominee carried the county in every presidential election from 1948 through 2004, except in 1992 when Bill Clinton won a plurality. In 2008, Barack Obama became the first Democratic presidential candidate to win a majority of votes in San Diego County since World War II; he won a majority of county votes again in 2012. In 2020, the county voted in favor of the Democratic presidential nominee Joe Biden by 22.8%, the largest margin for a Democrat since 1936.
The city of San Diego itself is more Democratic than the county's average and has voted for Democrats in each presidential election since 1992. Various cities within the county are swing areas that have split their votes in elections since 2000. Republican strength is concentrated in North County, East County and the eastern backlands. Coronado has also traditionally been a Republican stronghold.
One unique feature of the political scene is the use of Golden Hall, a convention facility next to San Diego's City Hall, as "Election Central." The County Registrar of Voters rents the hall to distribute election results. Supporters and political observers gather to watch the results come in; supporters of the various candidates parade around the hall, carrying signs and chanting; candidates give their victory and concession speeches and host parties for campaign volunteers and donors at the site; and television stations broadcast live from the floor of the convention center. The atmosphere at Election Central on the evening of election day has been compared to the voting portion of a political party national convention.
On November 4, 2008, San Diego County voted 53.71% for Proposition 8 which amended the California Constitution to ban same-sex marriages, thus restoring Proposition 22 which was overturned by a ruling from the California Supreme Court. However the city of San Diego, along with the North County coastal cities of Del Mar, Encinitas, and Solana Beach, voted against Proposition 8. La Mesa was a virtual tie for Prop 8 support, while Carlsbad supported the referendum by only a 2% margin.