Pretty much the only true mass communist party in US history. The RU was a promising start with a few thousand members in the mid-to-late 70s but it never came close to CPUSA's 100,000+ members and millions of sympathizers.
Although the CPUSA didn't openly carry guns at rallies they definitely weren't radlib pussies. While I was reading Kelley's book I lost track of all the times that CPUSA members got into shoot-outs with the Klan–many of them quite deadly. They would fight the police too if they had too since being a communist in Alabama in the 30s was more or less illegal for much of the decade. They did a lot of great work organizing steel and coal workers in Alabama my grandfather took part in a coal strike likely organized by them
They were militant fighters for civil rights long before the civil rights movement itself really got going in the 50s-60s. Most of the black writers you'll ever read in an American lit class (Baldwin, Langston Hughes, Richard Wright) was published in the party papers (ex. The New Masses), often starting up their careers.
They were mostly able to deal with idpol issues without sinking into the morass of radliberalism. They identified very closely with American revolutionary heroes such as Washington, Lincoln etc. and that made it easier for them to go against the anti-communist fervor of the time. I think it was also important for helping the American proletariat under standing where they are in history and where things might go from here.
In the North they didn't get into as many violent struggles but they had more bourgeois democratic rights so they had far more street presence there. One interesting they did do there was they would wait for a foreclosure or a tenant getting kicked out of their apartment and once all their stuff had been moved outside of the pavement by court order they would move all their stuff back inside when the police weren't looking and then the cops would get frustrated and leave the tenant in peace.
I don't think its an exaggeration to say that the New Deal was a reaction to the party's influence and the CPUSA was the backbone of trade-union drive in the 30s that saw a a third of American workers join unions. I don't think it's a coincidence that when the AFL-CIO began kicking out CPUSA militants who had really built up the movement that the slow decline of America's unions began. Their efforts building domestic support for America's contribution to WWII, which was an anti-fascist war, is not to be underestimated either.
To answer your question they were stamped out brutally in the 50s (the whole HUAC/red scare debacle) but that wasn't the only reason for their decline. They had a major revisionist wind in the party (ex. Bowderism) before the crack-down and the infiltration of the party but Khrushchev's secret speech was really the nail in the coffin. I also have my doubts about the black nation thesis but it was understandable given the times and at least a more coherent way to deal with the problem then just shouting "stop being racist!"
Pretty much all the party militants left the party and the FBI even set-up fake anti-revisionist parties to undermine both the existing CPUSA and the attempts to refound the party. All of the history of American communism post-1956 is the story of attempting to refound the CPUSA, whether the protagonists were aware of this fact or not. Nobody has succeeded yet.
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