In the elections of 2000, surveys showed that among those calling themselves Christians, only 50% were registered to vote. Then, on election day, only half of the Christians who were registered actually went to the polls. The Christian vote has declined 10% since 1994.
Then we sit around and wonder why the policies of our country are less and less Christian. And remember, some races were won by 90 votes. A single congregation, and the efforts of a single pastor, could have made the difference.
One of the reasons that some Christians don't vote is that they are being told not to by some of their leaders. Some Christians feel it is more righteous not to vote when the slate of candidates isn't that great. They feel compromised, dirty, or even sinful by casting a ballot for someone with whom they disagree.
Now it is true that to vote for someone who will advance un-Christian policies, precisely because you want them to, while rejecting a better, viable alternative, is indeed sinful. But when you are faced with two candidates, neither of whom is perfect (surprise!), but one of whom is clearly closer in his/her convictions to the Gospel than the other, it is perfectly legitimate to vote for the better one.
Some mistakenly call this "the lesser of two evils." It is not. In this example, one is not choosing evil at all. Rather, one is choosing a good. The good is the reduction, as much as possible, of an existing evil.
A clear example arises with abortion. All abortions are currently legal. If one candidate wants to eliminate more abortions than the other one, my vote for the one who wants to eliminate more can be seen as an effort to reduce the evil of legal abortion, and a choice to reduce evil is precisely a good.
Now some Christians, not finding a candidate who is willing to eliminate all abortions, do not vote at all. It is a mistake, however, for these Christians to think they will be "tainted" by voting for an imperfect candidate. The vote is not a vote for canonization, nor is it a declaration that one agrees with every position the candidate takes. (The only way to do that is to vote for yourself!)
What then, is the vote? It is a practical exercise in leadership, by which we do our part to put people into office who can make some improvement in our country's policies. Both we and the elected official are obliged to make the maximum improvement possible at the moment. At the same time, nobody is morally bound to what is impossible, and it is perfectly legitimate to recognize the limits of what is possible.
Every abortion is wrong, and somebody else's sinful choice made them legal, not ours. No vote can end them all today. But a vote that can help reduce the evil is, in fact, a good.