Imagine where Christianity would be today if too many people in the past took the Dobson approach to people in positions of leadership. What if the original disciples would have stayed home just because they didn't like the way James was running things or the message Paul was preaching.
Disagreeing with our leaders doesn't give us the right to stay home or lay down. We need to do the best we can with what God gives us. Anything less represents a lack of faith and a shortage of trust.
I’d say that we are to act with wisdom, holding God foremost, and within the laws. Beyond that I don’t think we “have to” vote. If wisdom says “don’t vote for either of the schlubs” then we shouldn’t. I don’t think it’s likely that will be the case (I will do a write in if there is a Romney against Clinton election, but think I will pull the lever for Guilianni against Clinton if it comes down to that). I like Duncan Hunter personally.
I think God is showing us with the USA the absolute best humans can expect to do on their own. And while this try is better than any that has come before, it is still woefully short of what God’s will is for us.