Yeah, I agree. I think i'm being realistic about the amount of work it's going to take. But that's roughly what Bernie did, and the dude's almost 80 years old
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Regarding the "split vote" stuff, there's no split vote at the city level. Most cities are dominated by Democrats (which should tell you something about that party). If a left wing independent party was actually giving Democrats problems against Republicans at higher level seats, they'd be forced to ease up on the various anti-democratic restrictions designed to keep 3rd parties out of contention. I'm talking about fusion voting (as won by the Working Families party in NY), ballot petition requirements, etc. That would be an unambiguously good thing imo.
I agree that infighting shit is a challenge, but the alternative is the status quo. Effectively, the entire US left being held hostage by the neoliberals and the "donor class" in the Democratic party. As long as the Hillary Clintons of the world think the left has nowhere to go, the left will no influence over the party. Just look at the way they treat unions.