Harriman broke with the SLP during the acrimonious split of 1899, which was largely linked to the SLP's insistence on establishing competing socialist dual unions with the existing trade unions affiliated with the American Federation of Labor.
In 1900 during unity negotiations between the Eastern and Midwestern SDP organizations, Harriman ran for Vice Presidency of the United States on the Social Democratic Party of America ticket along with Presidential candidate Eugene V. Debs.