Supporting is not joining.
Join them. Progressives need to run for local office as Dems. Join Dem committees. Canvas for them. Get to know them. Check out “Run For Something” to get involved.
If you want real change it involves leaving the house and talking to people and being present and available.
You can also vote in their primaries for progressives, if you prefer the less impactful “support” route. But then you’re just supporting the people who are doing the joining.
You can’t really complain if there are no progressive or left Dems to support if you as a progressive/left person aren’t joining either.
Democrats are more closely aligned today. The work would be easier. But sure.
This is a consequence of the two party system. You could dismantle that, but I think dismantling would require running one of the parties.