Hiram Johnson in the context of "Bull Moose Party"

The Progressive Party, popularly nicknamed the Bull Moose Party, was a third party in the United States formed in 1912 by former president Theodore Roosevelt after he lost the presidential nomination of the Republican Party to his former protégé turned rival, incumbent president William Howard Taft. The new party was known for taking advanced positions on progressive reforms and attracting leading national reformers. The party was also ideologically deeply connected with America's radical-liberal tradition. After the party's defeat in the 1912 United States presidential election, it went into rapid decline in elections until 1918, disappearing by 1920. The "Bull Moose" nickname originated when Roosevelt boasted that he felt "strong as a bull moose" after losing the Republican nomination in June 1912 at the Chicago convention.

As a member of the Republican Party, Roosevelt had served as president from 1901 to 1909, becoming increasingly progressive in the later years of his presidency. In the 1908 presidential election, Roosevelt helped ensure that he would be succeeded by Secretary of War Taft. Although Taft entered office determined to advance Roosevelt's Square Deal domestic agenda, he stumbled badly during the Payne–Aldrich Tariff Act debate and the Pinchot–Ballinger controversy. The political fallout of these events divided the Republican Party and alienated Roosevelt from his former friend. Progressive Republican leader Robert M. La Follette had already announced a challenge to Taft for the 1912 Republican nomination, but many of his supporters shifted to Roosevelt after the former president decided to seek a third presidential term, which was permissible under the Constitution prior to the ratification of the Twenty-second Amendment. At the 1912 Republican National Convention, Taft narrowly defeated Roosevelt for the party's presidential nomination. After the convention, Roosevelt, Frank Munsey, George Walbridge Perkins and other progressive Republicans established the Progressive Party and nominated a ticket of Roosevelt and Hiram Johnson of California at the 1912 Progressive National Convention. The new party attracted several Republican officeholders, although nearly all of them remained loyal to the Republican Party—in California, Johnson and the Progressives took control of the Republican Party.