Well, that's the dispute, right? Why don’t we have more left-wing policy? Is it because Republicans win elections and oppose it or because Democrats are secretly crawling with saboteurs? Answer to that determines your approach.
Relatedly, I think a lot of leftists are truly mistaken about how popular their policies are. They believe if Dems took bold, leftists stands, they'd win overwhelming majorities, but they don't take those easy wins because they'd rather lose to Republicans than implement leftist policy. I think that is extremely far from reality (my view is that Dems lose winnable elections because they have leftist principles that they aren't willing to compromise on).
Those real disputes don't get discussed because leftists regard their view as established beyond any doubt and so obviously true that anyone who expresses disagreement with them is only pretending because of some corrupt (if they're bought off) or evil motive.