You know there are other offices than president right?

You want a further left president, you’re going to need to show that a lot of people want a further left president, by having a lot of further left politicians in state and local offices.

I’ve seen the Democratic Party put its thumb on the scale for centrists at the congressional level: Henry Cuellar. I’ve seen them pull the rug out from under progressives who manage to win the primary, also at the congressional level: Michelle Vallejo.

Progressives cannot do as you describe when the party shuts them out at the lower levels as well.