That's why you vote in the democratic primaries, to do that thing you just said. Then you prevent republicans from getting power in the general. It's not hard, but leftists would have to actually vote instead of just bitching online. You wonder why democrats don't cater to leftists? Leftists don't fuckin vote.
In twenty years Dems have only held a supermajority for like two months and they passed the ACA. Fuck outta here with your bullshit.