I do not vote. I think I've talked about that before on this board (or one of it's predecessors), so I won't write an extended theological defense of that right now; maybe later, it there's more interest or I have more time than I have at the moment. Partly that is practical, for reasons you seem to grasp yourself, in trying to conscientiously support either major party; partly it allows me to maintain what you might call a "prophetic distance" and be critical, like I talked about before, without anyone being able to dismiss me as a partisan.
But mostly it is because I am deeply mistrustful of the church trying to co-opt the state to try and accomplish its goals. I think that misconstrues both our mission and our method. But we do not often think about that critically, because we think as Americans first and Christians second.
More to the point, I see a pretty standard false dilemma at work in some of what you said (and in Brisket's response, for instance). Political activism in a conventional sense and withdrawal are not our only two options. How is that being "in the world but not of the world?" The former looks pretty worldly; the latter is not in the world in any meaningful sense. But what if we really tried to live like Jesus as a community? As Stanley Hauerwas so profoundly put it, "The church does not have a social strategy; the church is a social strategy." We are supposed to be an alternative polis; "Jesus is Lord" was a radical political statement in the first century world. But it's much easier to try to compel people than it is to walk the way of the cross.