California was a leader in the Progressive Movement from the 1890s into the 1920s. A coalition of reform-minded Republicans, especially in southern California, coalesced around Thomas Bard (1841–1915). Bard's election in 1899 as United States senator enabled the anti-machine Republicans to sustain a continuing opposition to the Southern Pacific Railway's political power in California. They helped nominate George C. Pardee for governor in 1902 and formed the "Lincoln–Roosevelt League". In 1910 Hiram W. Johnson won the campaign for governor under the slogan "Kick the Southern Pacific out of politics." In 1912 Johnson became the running mate for Theodore Roosevelt on the new Bull Moose Party ticket. By 1916 the Progressives were supporting labor unions, which helped them in ethnic enclaves in the larger cities but alienated the native-stock Protestant, middle-class voters who voted heavily against Senator Johnson and President Wilson in 1916.

Political progressivism varied across the state. Los Angeles (population 102,000 in 1900) focused on the dangers posed by the Southern Pacific Railroad, the liquor trade, and labor unions; San Francisco (population 342,000 in 1900) was confronted with a corrupt union-backed political "machine" that was finally overthrown following the earthquake of 1906. Smaller cities like San Jose (which had a population of 22,000 in 1900) had somewhat different concerns, such as fruit cooperatives, urban development, rival rural economies, and Asian labor. San Diego (population 18,000 in 1900) had both the Southern Pacific and a corrupt machine.