Back in the 30's a lot of people were socialist and even communist in the US. The peak of this was in 1945 when our allies the Soviets essentially defeated the Nazis. But then came the Red Scare but also the brutal invasions and occupations as the Iron Curtain went up. That had a huge effect on the left in the US. And I think it continues today, with stuff like Russia's invasion of Ukraine. shooting down commercial airliners, China's genocide against the Uighurs, etc. Nobody wants to be THAT socialist. The communists in the US are pretty much just a few mentally ill folks who thrive on the notoriety. The remaining left are union-supporters, employee-owned company supporters, now medicare4all supporters: In other words, FDR-style Democrats. Just a little bit left of centrist Democrats.

So the "left's" positions are not hard for the rest of the Democrats to agree with. They just think that they might not be able to implement them, because half the country is still scared of "socialists coming for their kids and their guns". I don't think centrists are the left's enemies: I think they are just practical. Convince the majority of voters that medicare4all is good, and the Democrats will do it.