So as said here what are the best Points of divergence for a communist America
There's probably a lot pre-1900 but I'd actually point at joining World War I in 1914 as the best one. Let the war go on long enough that a serious anti-war movement can emerge, the anti-war forces in OTL ended up being some of the founding forces of the Communist Party. From there I think what would be key is to have a non-Moscow man in charge of the CP, so James Cannon securing leadership of the party would be a good second step. The final major change would probably be to keep the IWW healthy. It was always the element of the communist movement which most had its finger on the pulse of what Americans responded to and would serve as a key organizing element of the communist party.
From there the course of the timeline could be constructed to put the communist party (established with stronger foundations) into power in the 1930s as a consequence of the Great Depression.
How would you get the US in though?
Third term for Teddy Roosevelt is the easiest way. Dude was probably Americas greatest warmonger, and in the context of things would probably push for intervention just to get something out of it.