RFK, Jr. is a nightmare for Democrats. AND, it is willfully stupid to say that he will take votes equally from both Biden and Trump. The truth is, Trump voters are very unlikely to change to the hardcore Lefty RFK, Jr. Certainly, a few will, but that is a pretty rare bird. I mean, why would a person go from voting for a center-right candidate to voting for an extreme Left one?
It really is Lefty wish-fulfillment and gaslighting to say otherwise. It is just whistling past the graveyard. And it makes no logical sense.
OTOH, hard-edged Lefty voters are in general pretty disgusted by Senile Joe™ and thus are FAR more likely to shift. Conservatives were never going to vote for him, anyway. RFK, Jr. is much more in line with a hard Lefty worldview.