From 1901 to the onset of World War I, the Socialist Party had numerous elected officials throughout the United States. There were two Socialist members of Congress, Meyer London of New York City and Victor Berger of Milwaukee (a part of the sewer socialism movement, a major front in socialism, Milwaukee being the first city (and the only major one) to elect a socialist mayor, which it did four times between 1910 and 1956); over 70 mayors, and many state legislators and city councilors. Its voting strength was greatest among recent Jewish, Finnish and German immigrants, coal miners, and former Populist farmers in the Midwest. From 1900 (before its formal union) to 1912, the Socialist Party ran Eugene Debs for president at each election. The best showing ever for a Socialist ticket was in 1912, when Debs gained 901,551 total votes, or 6% of the popular vote. In 1920 Debs ran again, this time while imprisoned for opposing World War I, and received 913,693 votes, 3.4% of the total.
Early political perspectives ranged from radical socialism to social democracy, with New York party leader Morris Hillquit and Congressman Berger on the more social democratic or right wing of the party and radical socialists and syndicalists, including members of the Industrial Workers of the World (IWW) and the party's frequent candidate, Eugene V. Debs, on the left wing of the party. As well there were agrarian utopian-leaning radicals, such as Julius Wayland of Kansas, who edited the party's leading national newspaper, Appeal to Reason along with trade unionists; Jewish, Finnish, and German immigrants; and intellectuals such as Walter Lippmann and the Black activist/intellectual Hubert Harrison. The party outsourced its newspapers and publications so that it would not have an internal editorial board that was a power in its own right. The result was that a handful of outside publishers dominated the published messages the party distributed, and agitated for a much more radical anti—capitalistic revolutionary message the party itself tolerated. The Appeal to Reason newspaper thus became part of its radical left-wing, as did the Charles H. Kerr Publishing Company of Chicago, which produced over half of the pamphlets and books that were sold at party meetings.
The party had a tense and complicated relationship with the American Federation of Labor. The American Federation of Labor leadership, headed by Samuel Gompers, was strongly opposed to the SPA, but many rank and file unionists in the early party of the 20th Century saw in the Socialists reliable political allies. Many moderate Socialists, such as Victor Berger and International Typographical Union President Max S. Hayes, urged close cooperation with the American Federation of Labor and its member unions. Others in the Socialist Party's ranks dismissed the American Federation of Labor and its craft unions as antiquated and irrelevant, instead favoring the much more radical IWW and the syndicalist path to socialism.
In 1911, IWW leader William "Big Bill" Haywood was elected to the National Executive Committee of the Socialist Party, on which American Federation of Labor partisan Morris Hillquit also served. The syndicalist and the electoral socialist squared off in a lively public debate in New York City's Cooper Union on January 11, 1912, with Haywood declaring that Hillquit and the socialists ought to try "a little sabotage in the right place at the proper time" and attacked Hillquit for having abandoned the class struggle by helping the New York garment workers negotiate an industrial agreement with their employers. Hillquit replied that he had no new message rather than to reiterate a belief in a two-sided workers movement, with separate and equal political and trade union arms. "A mere change of structural forms would not revolutionize the American labor movement as claimed by our extreme industrialists," he declared.
The issue of "syndicalism vs. socialism" was bitterly fought over the next two years, consummated by "Big Bill" Haywood's recall from the SPA's NEC and the departure of a broad section of the left wing from the organization. The memory of this split made the intra-party battles of 1919-1921 all the more bitter.
The party's opposition to World War I caused a sharp decline in membership. An increase in the membership of its language federations from areas involved in the Bolshevik Revolution proved illusory, since these members were soon lost to the Communist Party.
The party also lost some of its most prominent members, who had been in favor of America's entry into World War I, including Walter Lippmann, John Spargo, J.G. Phelps Stokes, and William English Walling. They briefly formed the National Party, in an unrealized hope of merging with the remnants of Theodore Roosevelt's Progressive Party and the Prohibition Party.
In June 1918 the Party's best-known leader, Eugene V. Debs made an anti-war speech calling for draft resistance; he was arrested under the Sedition Act of 1918, convicted and sentenced to serve ten years in prison. He was pardoned by President Warren G. Harding in 1921 after a multi-year lobbying effort by the party and other labor activists.
By 1918, the Socialist Party of America had won 1,200 political offices, including electing 1 Congressman, 32 state representatives, and 79 mayors.