

It is a morally greater objective to turn these people around, and more practically achievable, than undermining its opponent.
The task is to build a mass movement to the left of Democrats. Are you going to have better luck building that out of the many Democrats who are disillusioned by the ineffectiveness of their party? Or is it going to be easier to bring in the folks cheering seig heils at Republican events?
Bernie had plenty of problems, but he would have been a significant step left of the party and he came pretty close to winning the 2020 primary. It’s easy to forget the unique circumstances that allowed Democrats to pull off the fix: a highly popular former president to coordinate the drop out, a Republican opponent with an unprecedented ability to rile up the Democratic base, and Covid hitting right before Super Tuesday. Without any one of those things, I’m not sure they could have slipped in Biden. Imagine if that primary had played out in, say, 2004.
You can also look at local elections – progressive District Attorneys, for example. They’ve ran (and a number have won) as Democrats, but significantly to the left of the mainstream Democratic candidates in their cities. And they’ve won by specifically appealing to voters who are moving left faster than Democrats on criminal justice issues.