Personally, I worry the most about RFK Jr. for the obvious reasons considering his family, but also because of what he represents to the right. RFK was a hardcore Democrat like the rest of his family but RFK actually stood up to the party when he felt they were going down a dark path. They refused to listen to him so he talked to the Republicans leading to him joining the party. The great thing is that RFK never actually changed his views or intentions, the Democrat party is what changed.
You see how violently mad they get about RFK when he's in those hearings and it's unhinged. It's hard to understand how they can be so mad over such simple shit like being cautious about what vaccines you take, and then you see Charlie Kirk get murdered simply for going into enemy territory and extending an olive branch. RFK is a living olive branch to Republicans.
I worry about Tulsi Gabbard in this regard too, but I haven't seen any unhinged rage directed at her from the left like I have RFK. But that might be because she hasn't given them an opportunity to be unhinged at her yet.