In January 1919 Vladimir Lenin invited the Industrial Workers of the World and the radical wing of the Socialist Party to join in the founding of the Communist Third International, the Comintern.
The Left Wing Section of the Socialist Party emerged as an organized faction early that same year, building its organization around a lengthy Left Wing Manifesto authored by Louis C. Fraina. This effort to organize in order to "win the Socialist Party for the Left Wing" met with staunch resistance from the "Regulars" who controlled a big majority of the seats of the SPA's governing National Executive Committee. When it seemed certain that the 1919 party elections for a new NEC had been dominated by the Left Wing, the sitting NEC, citing voting irregularities, refused to tally the votes, declared the entire election invalid and in May 1919 suspended the party's Russian, Latvian, Ukrainian, Polish, South Slavic, and Hungarian language federations, in addition to the entire state organization of Michigan. In future weeks, the state organizations of Massachusetts and Ohio would similarly be disfranchised and "reorganized" by the NEC, while in New York and Pennsylvania, the "Regular" State Executive Committees undertook reorganization of Left Wing branches and locals on a case-by-case basis.
In June 1919, the Left Wing Section held a conference in New York City to discuss their organizational plans. The group found themselves deeply divided, with one section, led by NEC members Alfred Wagenknecht and L. E. Katterfeld and including famed radical journalist John Reed favoring a continued effort to gain control of the SPA at its forthcoming Emergency National Convention in Chicago, to be held at the end of August, while another section, headed by the Russian Socialist Federation of Alexander Stoklitsky and Nicholas Hourwich and the Socialist Party of Michigan seeking to wash their hands of the Socialist Party and immediately move to the establishment of a new Communist Party of America. Eventually this latter Federation-dominated group was joined by important Left Wingers C. E. Ruthenberg and Louis Fraina, a depletion of Left Wing forces which made the result of the 1919 Socialist Convention a foregone conclusion.
Regardless, the plans of Wagenknecht, Reed & Co. to fight it out at the 1919 Emergency National Convention continued apace. With the most radical state organizations effectively purged by the Regulars (Massachusetts, Minnesota) or unable to participate (Ohio, Michigan), and the Left Wing language federations suspended, a big majority of the hastily elected delegates to the gathering were controlled by the Executive Secretary Adolph Germer and the Regulars. A group of Left Wingers without delegate credentials, including John Reed and his sidekick Benjamin Gitlow, made an effort to occupy chairs on the convention floor before the gathering was called into order. The incumbents were unable to block the Left Wingers at the door, but soon called the already present police to their aid, and the officers of the law obligingly expelled the boisterous radicals from the hall. With the Credentials Committee firmly in the hands of the Regulars from the outset, the outcome of the gathering was no longer in doubt and most of the remaining Left Wing delegates departed, to meet with other co-thinkers downstairs in a previously reserved room in a parallel convention. It was this gathering which established itself as the Communist Labor Party on August 31, 1919.
Meanwhile, elsewhere in Chicago the Federations and Michiganders and their supporters established the Communist Party of America at a convention gaveled to order on September 1, 1919. Unity between these two communist organizations was a lengthy and complicated process, formally taking place at a secret convention held at the Overlook Mountain House hotel near Woodstock, New York in May 1921 with the establishment of a new unified "Communist Party of America."
A Left Wing loyal to the Communist International remained in the Socialist Party through 1921, continuing the fight to bring the SPA into the ranks of the Comintern. This group, which opposed the underground secret organizations which the Communist Parties had become, included noted party journalist J. Louis Engdahl and William Kruse, head of the party's youth affiliate, the Young People's Socialist League, as well as a significant segment of the SPA's Chicago organization. These left wing dissidents continued to make themselves heard until their departure from the party after the convention of 1921.