For most of the 20th century, Sonoma County was a Republican stronghold in presidential elections. From 1880 until 1988, the only Democrats to carry Sonoma were Winfield Scott Hancock in 1880, Grover Cleveland in 1888 and 1892, Woodrow Wilson in 1912, Franklin D. Roosevelt in 1932 and 1936, and Lyndon B. Johnson in 1964. Like the rest of the Bay Area, it has since become a Democratic stronghold.
The last Republican to win a majority in the county was Ronald Reagan in 1984, and the last Republican to represent a significant part of the county in Congress was Representative Donald H. Clausen, who left office in January 1983.
On November 4, 2008, Sonoma County voted 66.4% against Proposition 8 which amended the California Constitution to ban same-sex marriages.
According to the California Secretary of State, as of February 2019, there are 277,665 registered voters in Sonoma County. Of those, 143,054 (51.5%) are registered Democratic, 49,386 (17.8%) are registered Republican, and 70,244 (25.3%) declined to state a political party. Every city, town, and the unincorporated areas of Sonoma County have more registered Democrats than Republicans.