Democrats aren't attacking Jill Stein because they think she is taking votes from Kamala Harris. No one I know who's voting Green would consider a vote for Harris at this point. They're attacking Jill Stein because they don't want voters to know that there can be a worker-centered party to the left of the Democrats that supports popular policies like Medicare for All, a $25 wage and federally guaranteed housing.
There are 80+ million eligible voters who don't vote at all because they don't see the point. Democrats are okay with this, in fact, they don't want any candidate to their left to appeal to those voters with popular policies.
The fact that the Green Party exists shows that the Democrats aren't pushing the most progressive policies. Jill Stein's candidacy shows that it's possible to support reproductive justice AND be against funding and arming a genocide. That we can end homelessness if we stopped funding endless wars around the globe.
Democrats don't want anyone to the left of them to exist because it's the only way they can convince Americans that Dem policies are "the best that we can do". To Dems, anything else is just "asking for a pony".
Don't fall for it. Despite Dem's desire to have you think otherwise, things don't have to be this way.
Another world is possible.
Sooner or later the us regime is going to collapse. It's fundamental tally unsustainable and just a good few crises away from imploding. And the best part is, you're looking right at the face of pretty much every kind of crisis imaginable. We've got climate crises, disastrous weather events, fascist coups, exploding inequality, de-dollarisation, the defeat of american imperialism in west and east Asia, mass immigration and so on.
So the violent overthrow of the American government is actually more realistic than trying to improve the Democrat party from the inside. Certainly the former will happen way before the latter.