I do prefer the model of the former OTL Democratic populists and People's Party constituencies and even politicians flowing into the Socialist Labor Party. I would like the SLP by World War II to resemble an oddly-agrarian, anti-war, and syndicalist social democratic party of the classic mold of that era.
Here was my model. Daniel DeLeon, in OTL the leader of a very small, nearly completely German immigrant socialist party (which, incidentally is the only highly-organized party of European First International politics, incorporating both revolutionary, anti-reformist Marxism and developing currents of revolutionary and anarchist syndicalism; it was founded in 1876 as the Workingmen's Party of the United States), is persuaded from his IOTL dictatorial party-leadership policies and harsh sectarianism by the fleeting result of late-coming Owenite utopian-colonist delegates to the Social Democracy of America convention, resulting in a more thoroughly pro-industrial unionist, party social democratic consensus. There is strong dialogue between the major constituent, the former Railway Union's Eugene Debs and DeLeon, which leads to the SDA Left folding into the Socialist Trade and Labor Alliance and the Socialist Labor Party of America. DeLeon relaxes his sectarianism and dictatorialism to permit this massive gain.
Subsequently, industrial unionists in the SLTA, which soonly allows dual-unionism and the participation as a minority faction of anti-partyist anarchist and revolutionary syndicalists, as well as certain autonomy from SL party policies, with SLP-SLTA major policy determined by both the party and union full leadership.
The Democratic populists and late People's Party constituencies and some leaders manage to temper their nativism and Christianity, and much of the already existing constituency mixing turns to full force. As IOTL former Populists went on to migrate to Canada, and adopted various brands of prairie socialism. IOTL, former People's Party constituencies turned to Debs' SPA.
In addition, the populist Republican progressives are also being forced out by the conservatives; some are turning to a kind of modernizing, corporativist, rationalization/efficiency-based, right-wing mangerialist reform. These guys end up pushing the anti-machine politics, municipal modernism, pro-elitism. They are for Wilson's Congressional Government and semi-presidentialism (which I think we need to strongly zero in on as being developed), and for international power politics. In the 20s, some of these guys will become adorers of Mussolinian fascism.
The more left-wing, populist reformers of the branch need to move into the SLP. I was considering the Social Democratic Party of America (SDPA) of the last update might serve as a kind of half-way house to the SLP of 1914; with Sewer Socialists and Left Progressives allying.
Eventually, both the populist Democrats and former People's Party, as well as left Republican Progressives and former sewer socialists, move into the SLP as a semi-reformist caucus. The leadership, the majority of the mass membership, and the official party program remain committed to industrial unionism, and socialism, but maintains programmatic endorsement of electoral politics and reforms as a minimum program and party strategy. However, the big-tent allows for a significant amount of local autonomy, delegation to local party organizations, freedom for fusionist tactics in electoral races, and intraparty federalism, as well as party-union federalism. The SLPA of 1914 is very much the social democratic party of the Second International, but it is very American-peculiar in character, owing to its unique birthing circumstances.
The war radicalizes and serves to further amalgamate the sewer socialists, left progressives, and populists into the SLPA. There is some fraying in the coalition by here-and-there defections by the fringes of the coalition to pro-war stances, but the anti-war stances serves mainly to create the new fundamental identity of the SLPA. Furthermore, nativism and rural antagonism for socialism and laborism dissipates substantially in this period (though some fringes obviously manifest in the formation of quasi-fascist, radical right groups in the 1920s, some of which espouse nativist and anti-urbanist rhetoric).
However, I do feel there's insufficient time for by 1919 the big-tent, anti-war, American Social Democratic SLPA to move seamlessly into the Comintern, especially without substantial butterflies. I had an idea that there could be an anti-Leninist split that joins the so-called "2 and 1/2 International" or Vienna International (formally, the "International Working Party of Socialist and Labor Parties", that was a kind of left reformist/left-of-reformist Leninosceptic/Lenin-critic socialist international) that I was calling the "Independent Socialist Labor Party of America" (basically, in pattern with the "left social democrat" Independent Social Democratic Party of Germany, that also co-founded IOTL the 2 & 1/2 International), that didn't accept the 18 Conditions but accepted socialism. Basically the small reformist-leaning and pro-coalitionist edge of the SLPA. Unlike Europe, the ISLPA will be less than half the size of the WPUSA throughout this period (the WPUSA will remain strong even among prairie socialists, which developed a substantial vociferously pro-syndicalist, anti-war left-wing, and that self-consciously fashions itself as the official organization of the 'small farmer' half of the "alliance of workers and small farmers" so championed by the party as its constituency). The ISLPA would serve as a kind of half-way house to the "pro-revolutionary consensus" to the "WCP-with-organized-internal-factions and the 'Christian socialist' Left Democrats" that is the dominant post-revolutionary feature of party politics.
In essence, during the "Second American Bienno Rosa", the Independents grow (but remain much smaller than communists) as the independent right of the socialist left (with the first draft's Syndicalist Group in the Solidarity Union representing the left-of-Communists niche for organized anti-party and left-of-Communist syndicalism). They are also the source of the WP leadership's "pragmatic break" with the Comintern in favor of a united front for a socialist minimum program and constitutional amendments. During the Revolution, the ISLPA splits into a "revolutionary left" and "reformist/conciliatory right"; the Left ISLPA provides a "right-of-Communist" opposition in the early soviets and socialist institutions, while the Left Communists, council communists, libertarian Marxists, revolutionary and anarcho-syndicalists, and anarchist communists provide the "left-of-Communist" opposition. The Right ISLPA becomes the bridge for the nascent Left Democrats into socialism and into Solidarity, etc.; the right ISLPA formally joins the Left Democratic Party after it identifies as a socialist party, and the Left ISLPA votes to fold into the Workers' Communist Party after the cessation of the Red Terror and the worst of totalitarianism and anti-pluralist vanguardism (as does most of the "partyist ultra-left" like hardline Trotskyists, Left Communists, as well as council communists, libertarian Marxists, and former anarchist libertarian socialists who have come around on parties), and with the official institutionalization of intra-party factions. Oddly, by the WCP voting for intra-party pluralism and looking to co-opt its revolutionary socialist competition, it destroyed itself as a distinct entity: these intra-party factions and policy clubs coalesce into two major intra-party coalitions-of-factions-and-clubs, into a broad ultra-left-wing (the left of the Old Bolsheviks together with the ultra-left entryists/coopteds), and moderate-conservative Leninist wing (former Stalinists with the absorbed Left ISLPA) that in 1946 vote to accomplish the obvious reality, and form the Socialist Party and Progressive Labor Party, respectively.
That's my broad framework for the formation of the mature party system; I would like to draw contrasts with party-formation, party-organization, intra-party decision-making and centralism, as well as programmatic and electoral politics, from European Social Democracy. I would also rather the "liberal left reform" politics are co-opted into socialism, rather than remaining an independent current (this will certainly lead, IMHO, into the formation of a class-collaborationist, fundamentally status quo party looking to civilize capitalism and for its leadership to join the policy and decision-making elite/establishment complex, just like right-wing Social Democracy in Europe). Rather, I submit this cyclical system of repeated hardline anti-coalition and increasingly coalitionist politics, which prevent the American left from being ever successfully coopted into the Establishment in its majority. Furthermore, there is a cross-pollination between socialism and Establishment reform/left-split politics, that eventually produces all of the major parties prior to the Second Cultural Revolution: Workers' Communist Ultra Left-Socialist Party, Workers' Communist Center-Progressive Labor Party, and the Left Democrats.
EDIT: Besides the formation of a broad and powerful Communist Party in the United States which seriously poses and assumes the question of power in a revolutionary scenario in 1933, using our 1890-1910 tools here isn't my only purpose: I also made a lot of the above modelling on the assumption that even a milder and broader and more moderate Comintern and Leninism, and a slower and milder progression of Stalinism within the USSR (and by extension, in the Comintern itself), would involve a certain intensification of generalized sectarianism within the socialist left both on an international and domestic scenes in the 1918-1933 period. The difference is the various sects and splits from the Comintern all remain very marginalized and small or highly localized, with the exception of the Independents, who unlike European Social Democrats, never get more than a quarter of the WP support, I figure and remain a subsidiary force.
