Why are establishment Dems afraid of the 'hard left'?

[Proud Liberal Dem]

39. I'm frequently mystified about the double standard for the two parties

Republicans can be as spun out right-wing as they want, but Democrats are repeatedly cautioned to ignore or sideline progressives like AOC lest they become unelectable to "middle-of-the-road Americans (whom I'm not sure care as much as pundits claim they do). Republicans, however, can be as anti-immigrant, anti-POC, anti-poor, etc. and they are repeatedly elected and re-elected without consequences but the instant we have Democrats argue for universal health care, argue for sensible gun restrictions, argue for measures to help combat climate change, argue for reproductive choice and against draconian anti-choice measures, suddenly, they're getting "too extreme" and won't be elected. Although what we've seen in practice often suggests that, in their rush to avoid a HL label, many Democrats, especially in more Red States, openly denounce progressives and veer too far to the right to compensate, often with disastrous results (i.e. Claire McCaskill being my favorite example).