I'd like to try and understand why they think voting third party would be more effective in improving people's day to day lives than voting for a "capitalist" party. Organizing, serving vulnerable populations, these are wonderful things, of course, and absolutely they will improve lives. But how does the third part vote itself help? In my mind, it seems more likely to hurt, especially in the short run in our current system.
It's one thing if it were an attempt to hold the democratic party hostage. In other words, withholding their vote until some certain policy priorities are changed. If the block was large enough, that would force the dems to decide if making those (presumably progressive) changes would gain enough votes to make up for the voters they might lose for enacting them. I don't necessarily agree with that tactic, but I can see the logic behind it. But that seems to be not what you are arguing for at all. You aren't trying to change the dems, you are trying to create something new entirely. And I just don't see how throwing away votes now, which again, to me seems more likely to hurt vulnerable populations in the short run, is helpful to that end.