I agree... tentatively. I haven't yet spent much time considering the idea of acausal influence in its most general form, but I'm not sure I see how it would apply here; you can have some pre-election influence by virtue of what sort of person you are (or seem to be), but when it's election day, it seems like you should be able to decide to vote or not vote without your decision retroactively implying too much about what other things you could have caused.

I realize that sounds exactly like the argument for two-boxing, but I'm not convinced the causal structure is similar enough for the analogy to be valid.

(I've previously had vaguely relevant thoughts about the expected payoff of one vote. I should expand on that at some point...)