one pretty fundamental - yet extremely problematic - form of moral reasoning that people will use to corner you into the hillary/donnie dichotomy. alright you're going to vote for stein (johnson). what if everyone thought like that? then trump (clinton) will win! in other words, you're not only responsible for your own vote in the real world, which has almost no effect, but for other people's hypothetical votes in a possible world.
of course, if everyone thought like me, everyone would be voting for something much closer to their convictions and the political system would pop open like a carnivorous plant, so let's not worry about that. but what is the argument? that i might be persuasive to people? well, that is not a result of my vote per se. that 12.3% of voters are watching over my shoulder as i vote and will imitate me?
in general (i am talking to you, immanuel kant), i think we would do better to think about the actual principle of our own actions and the actual effects of our actual actions in the actual world. being responsible for that is more than enough, and the only thing that has you reasoning about reality. i would try to show how such an approach would not necessarily be sheer individualism or environmentally disastrous, etc. but whether i can or not, you had better have an argument handy that i should be responsible for the non-actual effects of my action in a non-actual world.