They are all locked into the "both sides are the same" frame of thought. Democrats to them, no matter how progressive, are all tools of the bourgeoisie and running dog lackeys of American imperialism.
If any of them claim to be Democrats it's only for the purpose of trying to infiltrate or co-opt us.
Back in the 1930s and '40s groups like these were very much larger, and more influential, and actually did some good work. The CPUSA for instance took the lead in defending "the Scottsboro Boys" from being murdered for a crime they obviously didn't commit, and the SWP in the 60s did some antiwar organizing that was helpful to the overall cause. But the Hitler-Stalin pact of 1939 and Khrushchev's speech denouncing Stalin in 1956 took a lot of wind out of their sails, so by the '70s they had shrunk to insignificance. Then too the FBI under Hoover and its COINTELPRO did a lot to undermine and eventually destroy many of the more radical groups. There's a report I read years back, for instance, that claimed that by the mid 1970s fully one third of the entire SWP membership were FBI informants. The fall of the Soviet Union pretty much was the final straw.
The radical right, as you say, is so much more powerful, and has in fact seized control of the GOP. But then the radical right has always had so much more power than the left, having as it always had backing from the extremely wealthy and much of mainstream religion. Hell, they even for a while had their own country within a country--the Confederate States of America. And now they have control of the Republican Party.
I'm not at all looking forward to the next four years.