What was the relationship that the Socialist Party of America had with communities of Color in the earlier 20th century? Was there attempts by Debs and other leading Socialists to attract Black or Mexican or Asian supporters across the country?

I first want to disclose that I am not a historian but I am someone who has read a lot about American socialist and labor history.

Before I directly answer your question, I'll first have to make a brief (wildly over simplified) digression into Marxist theory. It will prove illuminating to understanding the SPA's relations with non white peoples in the US.

Prior to the Bolshevik Revolution, mainstream Marxist theory (much less so Marx or Engels themselves) narrowly understood industrial society to be divided between the bourgeoisie - those who own property and purchase labor - and the proletariat - those who own nothing but their labor, which they sell. Initially, this model applied primarily to England (since this was the real world case off which the model was actually based), but it soon became practically useful for regions in Continental Europe which had fallen to Capitalism. Adjustments to the theory were made where national conditions made it necessary (e.g. Prussia with its Junker landed aristocracy), but the same general outline applied. Indeed, the model worked pretty well for Western/Central European regions, but quickly fell apart when taken out of this context - which is what the SPA did, partially shaping their views on non white minorities (more on that below).

Despite the United States's status as the archetypal Capitalist country, traditional Marxist theory fails to work here since the US is not Western Europe. This sounds trivial, but it's something the SPA (and many other contemporary and later American socialist movements) completely failed to understand. They applied Marxist theory (as developed by the likes of the SPD in Germany) without adjustment to the US and its particular historical/social conditions, most important of which are settler colonialism and white supremacy. When faced with the reality that this country's working class is deeply stratified by race and that one part of it - those considered white - acted as a massive garrison force against the other part - those considered non white - the SPA either chose to ignore it (colorblindness) or to support it (open white supremacy, of which we'll see some examples below). Broadly speaking, the Party simply refused to understand the extent to which class in the US was (and remains) inflected by race, and the ways in which both function as manifestations of each other. There were notable outliers such as Hubert Harrison who was one of the few voices in the early 20th century to push for an active Marxist response to the "race question" in the US. He was the first (before even WEB DuBois!) to capitalize the n in Negro, indicating that Black folk were in fact their own people. But Harrison was a rare example of clearsighted and genuinely radical analysis in the otherwise white sea of the SPA. Indeed, he would later be expelled from the Party for his stubborn insistence on championing non white struggles as equally valid as or even leading the labor movement. So what did the SPA actually say and do as regards non white workers?

Let's look at Eugene Debs, who was among the better leaders of the SPA, to get an idea. Debs first made his name in the American Railroad Union (ARU) and remained for many years a railroad unionist. We note that the railroads were unionized on a lily-white basis. So extreme was the white supremacy that there was a brief period from the 19th to the early 20th centuries where the Southern railroad unions waged 50+ strikes in order to force the expulsion or demotion of Black workers and to guarantee white monopoly in high paid/comfortable railroad jobs. Debs until his death claimed that there was no special race question and that Black and other non white workers' trials would be solved by the labor movement. To his credit, towards the end of his life, Debs refused to speak at segregated audiences - but he still stuck to his colorblindness. For example, on hearing of Thomas Watson's (leader of the Populist Movement in the 1890s) death in 1922, Debs had this to say:

He was a great man, a heroic soul who fought the power of evil his whole life long in the interests of the common people, and they loved him and honored him. [Cited in Sakai]

Debs failed to mention (or deliberately ignored) that after the failure of the multiracial (though still racially tense) Populist Movement, Watson became a raging white supremacist, who on one hand could praise the Russian Revolution, while on the other extol the "virtues" of lynchings against Black people alongside many other rabid white supremacist diatribes. It is worth noting that as a Populist leader, Watson actually bemoaned how white supremacy blinded white farmers from their interests and often defended Black farmers against white attacks and race baiting. His turnaround was therefore even more jarring to those who remembered his Populist days. Yet Debs remained totally unconcerned - and this was as one of the more "progressive" minded SPA leaders. The Party's right wing was even worse. Take this example:

In 1904, the Socialist Party executive sent the following response to the Second International Executive which had inquired about the issue of Ku Klux Klan lynching: "The Socialist Party points out the fact that nothing less than the abolition of the capitalist system and the substitution of the Socialist system can provide conditions under which the hunger maniacs, kleptomaniacs, sexual maniacs and all other offensive and now lynchable human degenerates will cease to be begotten or produced." [Cited in Kipnis]

How charming. The Party also operated segregated locals where locals were not all-white, something which Harrison tried to combat, and which contributed to his eventual expulsion from the SPA.

Attitudes towards immigrants were not much better. In 1911, to a speech to the House of Representatives, first term Congressman Victor Berger (leader of the Milwaukee SPA chapter and Debs's regular presidential running mate) described East European immigrants to the US as "modern white coolies" (he had also officially claimed that Black people were an inferior race). It should go without saying that this also betrays his attitudes towards the "old coolies", meaning Asian migrants to the West Coast. Indeed, immigration was something of an embarassment for the SPA - they were one of the very few parties in the Second International to be bitterly indifferent if not rabidly hostile to immigration. For example, in 1907, at the Second International convention in Germany, the Congress proudly endorsed unrestricted immigration and opposition to all border controls as stalwarts of international worker solidarity. The SPA was less enthusiastic: Morris Hillquit, noted Party leader and himself an immigrant from Latvia, proposed banning migrants according to nationality, which would allegedly serve as an indicator for their "capability" to be educated in class struggle. The vast majority of the Congress was utterly appalled and shot down the proposal, and would later officially oppose the border exclusion of any peoples. It's worth noting that the only delegations which supported Hillquit were those of South Africa (already home to a system of proto apartheid) and Australia (where the labor movement spearheaded then rabidly supported the White Australia policy). "We are the company we keep" and Hillquit's company were raging white supremacists. Still, the SPA did not learn its lesson and would experience bitter internal struggle over immigration for the next few years. Victor Berger and Ernest Untermann, both Central European immigrants, wrote up a proposal to exclude Asian immigration on the grounds that:

if we admit the Chinese, the Japanese, and the Korean. . . it will be clear to America's workingmen that the party does not want to help them. [Cited in Miller]

Outside of Berger and Untermann's own chauvinist views, this reveals two things: (1) that the SPA's idea of the American working class was only that part of it which identified as white; and (2) that this white working class was so virulently chauvinist that anything less than open white supremacy was considered politically risky by socialists. Perhaps Berger and Untermann were reading their own racism in white workers, but I find this questionable. Not ten years later did whites (most of whom workers) wage horrifying pogroms against Black people across the country during the Red Summer of 1919. Further still, the labor movement on the West Coast was the driving force behind the Chinese Exclusion Act of 1882.

NOTE: I'm not quite finished with my answer but I have to go for now due to other commitments. I'll finish later on. I've provided a list of references for the moment. Regards.

References:

J. Sakai, Settlers, Morningstar Press, 1989

Sally Miller, "For White Men Only: The Socialist Party of America and Issues of Gender, Ethnicity and Race" - The Journal of the Gilded Age and Progressive Era, Jul., 2003, Vol. 2, No. 3, New Perspectives on Socialism I (Jul., 2003), pp. 283-302

Josh Honn, "Coming to consciousness : Eugene Debs, American Socialism and the 'Negro Question'"(2002). Theses and Dissertations. Paper 747.

Herbert Hill, Black Labor and the American Legal System, University of Wisconsin Press, 1985

Ira Kipnis, The American Socialist Movement: 1897-1912, Monthly Review Press, 1952