It will probably be most effective to build locally: get plugged in on local labor fights and help build solidarity funds. Solidarity funds for small groups of workers are usually just cash funds managed by the workers themselves with zero administrative overhead.

Some of them might not even have a fund and you can help them out by starting one. Tabling at local events for their fund and recruiting others and so on. You'll amplify your impact a lot by fundraising from others and getting more people involved.