How many progressives do we have in the house? How many green party members (or any independent/third party member who wasn’t initially elected as a Democrat/Republican)?

If you’re a third party in the general, the deck is stacked against you even stronger than a progressive in the primary.

You could steal the green party but it’s not going to get you anything. You’d have to do the exact same thing of building up the party from local, to state, to federal offices. And you don’t get to drag along the people who are lifelong Democrats. You’d have to build up the green party and then take on both the Democrats and Republicans.