David Herreshoff’s American Disciples of Marx: From the Age of Jackson to the Progressive Era (1967) reveals the SPA as it appeared to the rival SLP’s Marxist theoretician, Daniel De Leon. De Leon viewed the SPA as a fundamentally electoralist and reformist party, Haywood and Debs notwithstanding. Berger and Hillquit hoped that by proclaiming neutrality in internal AFL affairs the SPA would more easily win the AFL leadership’s political support; this neutrality became official SPA policy by 1909. Believing this approach was mistaken, De Leon continued to develop his theory of dual unionism in response. While industrial unions organized toward their eventual control of the means of production, De Leon argued, they needed to support an uncompromising political party that would prepare to take possession of (and dismantle) the capitalist state. De Leon’s Socialist Trade and Labor Alliance followed this strategy when it merged to form the IWW in 1905, but by 1908 the IWW came to be dominated by anarcho-syndicalists who saw no need for a socialist party. This prompted De Leon to reconsider and embrace the Second International’s mass party model, which allowed both reformist and revolutionary tendencies to operate within a single party. After years of sectarian infighting, however, even Haywood and Debs were unenthusiastic about De Leon’s efforts, after 1908, to achieve SPA-SLP unity.
The British guild socialist G. D. H. Cole offers a comparative portrait of the SPA alongside other Second International parties in Volume III, Part II of A History of Socialist Thought: The Second International (1956). Cole argues that while the heavily German-American and immigrant SPA could look to the Social Democratic Party of Germany (SPD) as a model (with its informal political leadership of the German trade-union movement), the SPA never exercised comparable leadership of American trade-unions because the United States lacked the degree of political unfreedom that drove German unionists to embrace socialism as a party politics. While American socialists took great inspiration from their European counterparts, Laurence R. Moore’s European Socialists and the American Promised Land (1970) shows how European socialists from Marx to the Second International viewed American prospects for socialism. To revisionist Marxists, American trusts promised an inevitable evolution into socialism; meanwhile, American socialists’ disappointing electoral showings dashed some orthodox Marxists' expectations that intensifying class conflict in the United States would necessarily result in revolutionary party politics.
Most histories of the Second International pay little attention to the SPA’s role within it. In the absence of a monograph devoted to this topic, Sally Miller’s 1976 article, “Americans and the Second International,” remains essential. Compared to the SPD, the SPA was founded later and achieved far less influence within the labor movement, yet the two parties shared a fundamental similarity, Miller argues: they both condemned reformism in theory while embracing it in practice. The Second International failed to specify the proper relationship between the political party and the labor unions, beyond endorsing their cooperation as autonomous entities; Haywood’s model of politically uniting the IWW behind the SPA failed to win the International’s endorsement. Dominated by Hillquit and Berger, Miller shows, the SPA’s delegations to International congresses manifested right-wing tendencies: they evaded the International’s directive to negotiate unity with the SLP, supported Socialist participation in capitalist governments, favored state restriction of immigration based on the perceived “backwardness” (and initially on the race) of an immigrant’s country of origin, and stopped short of endorsing a general strike in the event of a capitalist war. Miller concludes that the SPA’s delegates to the Second International’s congresses primarily derived prestige, camaraderie, and theoretical self-justification from their participation.
Intellectual histories of Debsian socialism are dominated by the question of whether the SPA gave rise to a body of theory that could be described as genuinely Marxist. James T. Kloppenberg’s Uncertain Victory: Social Democracy and Progressivism in European and American Thought, 1870–1920 (1986) argues that the revisionist tendencies within Second International Marxism converged politically with non-Marxist progressive liberalism. Two books make the case for the triumph of progressivism within the intellectual life of the SPA: Mark Pittinger’s American Socialists and Evolutionary Thought, 1870–1920 (1993), which shows how American socialists drew reformist political conclusions from evolutionary scientific thought (trading Karl Marx for Charles Darwin and Herbert Spencer); and Brian Lloyd’s Left Out: Pragmatism, Exceptionalism, and the Poverty of American Marxism, 1890–1922 (1997), which shows how American socialists drew similarly reformist political conclusions from pragmatist philosophy (trading Karl Marx for William James). In the same vein, Donald Stabile’s Prophets of Order: The Rise of the New Class, Technocracy and Socialism in America (1984) explores the thought of SPA socialists-turned-progressives (such as Walter Lippmann and William English Walling) who envisioned novel technocratic roles for a rising class of expert professionals suited to the management of “collectivist” war planning. By contrast, Robert Hyfler’s Prophets of the Left: American Socialist Thought in the Twentieth Century (1984) profiles SPA left-wingers Eugene Debs, Louis Boudin, and Louis Fraina to find revolutionary Marxism at the root of their theoretical perspectives. Similarly, Anthony V. Esposito’s The Ideology of the Socialist Party of America, 1901–1917 (1997) argues that the SPA’s industrial-socialist wing fused a Marxian conception of class struggle to a republican critique of wage slavery, warranting the Party’s joint pursuit of workplace and electoral action as championed by Debs. It was only after 1912, Esposito argues, that this Debsian fusion broke apart into counterposed direct-actionist and electoralist wings whose members tended to leave the SPA for Wobbly industrial-unionism and Wilsonian liberal-progressivism, respectively. The final chapter of Matthew E. Stanley’s Grand Army of Labor: Workers, Veterans, and the Meaning of the Civil War (2021) documents how Debs framed the SPA’s struggle against wage slavery in abolitionist terms.
Other works address the relationship between Marxist theory, leftist intellectuals, and the consciousness of American workers (both native- and foreign-born). Aileen S. Kraditor’s The Radical Persuasion, 1890–1917: Aspects of the Intellectual History and the Historiography of Three American Radical Organizations (1981) explores the failure of Debsian socialism in Leninist terms as the failure of intellectuals to bring socialist consciousness to “John Q. Worker.” Kraditor contrasts various SPA radicals’ contemptuous perceptions of workers’ politics with evidence, synthesized from the new social history, of workers’ perceptions of their own private-life goals. She suggests that socialist ideology only found a brief political opening among American workers because the (typically ethnic) institutions that met workers’ needs under capitalism proved incapable of fully performing this task during the transitional period of the Second Industrial Revolution. Meanwhile, John Patrick Diggins’s The Rise and Fall of the American Left (1973) emphasizes the homegrown intellectual roots of Greenwich Village’s “Lyrical Left.” Like Kraditor, Diggins critiques Debsian socialism as a movement of intellectuals on behalf of the working class. While acknowledging the vitality of native-born socialists, Paul Buhle’s Marxism in the United States (1987) stresses the role of foreign-born socialists in bringing Marxism to America. The divide between the SPA’s electoralist and industrial-unionist wings, Buhle suggests, mapped onto the divide between the complacent Marxism of the established German-Jewish immigrant enclaves and the radicalized Marxism of the hardscrabble new immigrants attuned to mass strikes in Europe after 1905.
While most intellectual histories of the SPA focus on Marxism, the biographical essays collected in Socialism and Christianity in Early 20th Century America (1998) trace the thought of various Debs-era SPA members—among them Black preacher George Washington Woodbey, agitator and columnist Kate Richards O’Hare, Rand School of Social Science founder George D. Herron, and “millionaire socialist” J. G. Phelps Stokes—for whom Christianity served as a major impetus for their embrace of the SPA’s politics.
American Socialists and Evolutionary Thought, 1870-1920 by 
Socialism and Christianity in Early 20th Century America by 
