Well, people can and DO carry poison. I’ve done it personally. The point of speaking of it that way, is once one realizes that one is only carrying poison, and need not be poison, one can more confidently set the poison down and walk away. And be more hopeful about other people who are presently carrying poison.
I would probably be better off if I just describe more literally what I see to be poison and how it applies to the candidate in question than using the word symbolically.