You’re changing my premise.

OP is saying that you should vote for leftists or progressives. I’m saying that they don’t tend to have strong enough support in the American electorate to win primaries. You’re saying that leftist or progressive ideas win elections. That’s not the same thing at all.

Certain progressive ideas have reached some kind of national consensus, like the idea that minimum wage needs to be a living wage. That’s still not popular in a lot of places, but okay, let’s roll with it. OTOH, major structural criminal justice reform–reduction in police, more addiction treatment programs (that aren’t based in religion), reform-minded prison, etc.—does not have popular support. Republicans are largely law-and-order/crime-and-punishment types, and a significant percentage of Dems are as well. Running on a ‘defund the police’ platform tends to cost people elections, which is why you see Biden increasing funding for cops. It doesn’t matter if data strongly suggests that the approach we’re using doesn’t work, and can’t work; people that see crime as a moral issue can’t accept anything other than punishment.

If you want to change all of this, you have to start long before elections.