Yes form a new party to split the progressive vote so they never win any substantial seat. Great idea.
The kinds of seats that are realistically winnable right now are in cities. Cities are generally one-party states run by the Democratic party. There's no threat of splitting the vote with Republicans by challenging the Democrats for city council, most mayoral races, and even quite a lot of state congress seats. In fact, an absurd number of seats are run totally unopposed:
https://www.thenation.com/article/alaskas-lesson-left/#
Furthermore, many cities have "minority seat" provisions where a city council containing 10 Democrats, often set aside a couple seats for minority parties. If there's no challenge, these seats will go to republicans. So there are a lot of possibilities for alternative parties to take some meaningful power. The logic of "first past the post" is true at the presidential level, but decreases in severity the lower you go.
All this being said, I
oppose the Democratic party. The success of their policy preferences is the failure of mine. The success of corporate charter schools for example is the failure of a strong universal right to a good public education. So I absolutely reserve the moral right to challenge them electorally.
Unlike you I understand that accepting campaign money does not make someone beholden to those who contributed. Judge them based on policies and voting record, not perceived slights against the working class for accepting campaign funding.
I'm not going to strawman you here, but virtually no liberal Democrat holds this position when it comes to Republicans. Do you think the GOP stance of climate change denial has any substantive basis other than intense funding from the fossil fuel industry? If not, then why do you think Democrats are immune to this influence?
There's actually good academic work that completely disputes your "understanding":
https://en.wikipedia.org/wiki/Investment_theory_of_party_competition
