A few things that are accessible within the USA include:
- Participating in mutual aid programs
- Campaigning on the local level, including for positions like poll watchers
- Making your voice heard in community events in general
- Joining your local DSA, networking
I keep thinking that left-leaning Americans need to make a shadow party, shadow Democrats. They recruit people to join, they push for more progressive Democrat candidates, and try to push the more conservative candidates out. Then, when all the effort is done, they vote Democrat at the polls. In the current model, there is no way to succeed as a third party until one of the other parties collapses. So the only real options for change are: take over an existing party, wait for a party to collapse, or change the model (which requires a major nationwide effort to implement).
“The shadow Democrats” already exist, they are called the third-way except they pushed all the progressive/leftists out of the democratic party, and also are funded by groups that are threatened by progressive taxation, let along leftists policies.
So what you would like to happen cannot happen without funding, so a different approach has to be tried.