Let me take on that question, as someone who would willingly attack Hillary while simultaneously refusing to support Trump.
If I want to be internally consistent as a supporter of truth/justice/liberty/all those tasty things, I MUST NOT hesitate to release damaging information along those lines for the sake of a "lesser of two evils" justification. Your statement of zero-sum two person race is also a principle I believe one must sacrifice a "lesser of two evils" discussion to rally against, I believe that accepting we are in a two person race is part and parcel with the level of polarization/group-think we're seeing nowadays. The first steps to a better world in my eyes are to question EVERYTHING and become as vehemently anti-tribal as possible, and that means not withholding things that my moral code would demand I act on, even if it might harm the horse I'm rooting for. (quite to the opposite actually, if I saw that my main candidate had flaws and wasn't getting called out on them, I would see it as my obligation to do so even MORE so, since to whitewash in that direction to my eyes weakens what would otherwise be a valid stance. e.g., all things have negatives, even perhaps a positive choice, but to ignore them is duplicitous.)
tl;dr, my goals in this are not "this election" or "this candidate"; it's "getting people to question more"; "getting people to realize it doesn't NEED to be a zero-sum game". One may say this is idealistic/drop in the ocean etc; but I cannot reconcile with my personal philosophy giving up, because if I did, then there is no justification for ANYONE ELSE to push for these "impossible" goals, and thus the status quo/tragedy of the commons continues.