I don't mean electoral parties; I mean political parties in a broader sense than just a machine to get people elected to office, though parties may participate in electoral politics among other things. What I mean by a political party is an organization with an _explicit & comprehensive_ political program collectively worked out, agreed upon, adhered to, & promoted by its members (I exclude one-issue organizations from my definition of political parties, though other people's definitions may include them). You join a party when you agree to its program (including its long-term political objectives). If you agree to most parts of the program but not all, you may still join it & try to change the parts you cannot accept in good conscience. If the party fails to change its program in accordance with you political belief, you may leave it, to join another or build a new one with like-minded people; if you decide that disagreeable parts of the program are far outweighed by agreeable parts, you may stick to it.
What you are talking about in your post is movement-building, & many movements can come into being & even flourish without any political party at all. Feminist movements, GLBT movements, etc. are good examples of movements without political parties. A movement is broader than political parties; sometimes, political parties with radically incompatible long-term objectives may work together in a united front, focusing upon shared immediate goals, in the same movement.
As for unionism, one may be a union organizer without being a party organizer, and vice versa, especially in the United States where unions & parties don't have a close working relationship (compare the relationship between the Democratic Party and the AFL-CIO with that between the Japanese Communist Party with Zenroren, to take just one example).
Party membership doesn't prove any "activist street cred," and vice versa. You may be an inactive political party member. You may be a very politically active person and yet refuse to belong to any political party, perhaps out of a matter of principle if you are an anarchist, perhaps because you can't find any that you want to join. You may be a very active organizer of a political party to which you belong. You may even be a very loyal fellow traveller who tirelessly promotes a political party without belonging to it & "out-activists" party-members who are members in the name only.
Movement-building is far from irrelevant to party-building. In fact, the latter cannot happen without the former. However, a movement & a political party are not the same thing.
While I am politically active, I am not a member of any political party (in fact, I have never joined any political party). I think, though, that there should be one that a sizable number of leftists in the States can join. At least I wish there were.
Yoshie