The argument that you should always pick “the lesser of two evils” is, without a doubt, the most common and most specious piece of reasoning in the whole circus. This lethal bit of illogic is popular for a very obvious reason: it justifies voting under any circumstances and for any candidate. If you accept it, then you never have an excuse for not voting. End of story.
For example, if a guy suggests that both candidates in a particular race are unsatisfactory, he will receive the following retort:
All men are flawed, and so it is unreasonable to demand all your expectations to be personified in the candidates before you. And so, although you cannot have what you want, you can at least measure the two and pick the one you don’t want the least. You must choose the lesser evil. In fact, my dear boy, you are morally bound to do this. Otherwise, you will be responsible if the “greater evil” wins instead.
Infallible bit of reasoning, that.
Now, if you happen to be Christian, this is really easy to answer. Christians are forbidden under any circumstances to choose evil, even if good may come of it (Romans 3:8). End of story. No rhetoric, no sob stories about the greater good. A Christian may not choose evil, because to choose evil is to orient the will toward it in such a way that even if good comes in the long run, the will was still oriented toward an evil, and this is unacceptable. For a Christian, that is. And this remains true even if Christians, during election season, are running around spouting off about “the lesser of two evils,” just like everyone else. Their ignorance of Christian morality does not render it false. It isn’t Christ’s fault if so many Christians are stupid.
I mention the Christian aspect of the problem because so many Americans are in fact Christian, or at least that is the label they have appropriated for themselves. But even outside the Christian context, this argument fails miserably.
Practically speaking, “the lesser evil” argument would have us voting for Hitler himself if Stalin or Mao were the alternatives. In the end, the “lesser evil” argument is fallacious, and is only popular because it allows voters to justify themselves, and it enables them to bring moral pressure to bear on their neighbors who refuse to participate in evil.
In the end, all the argument does is guarantee that no desirable candidates will ever be produced by either party. The parties will not produce them because they will not have to. All they need to produce in order to win is a “lesser evil.” The candidate they offer will not be judged based on his virtues, by the standard of what is good, but on his opponent’s vices, by the standard of what is most evil. Within this structure, party candidates will slowly become less and less good, more and more evil, and the people will be morally obligated to continue voting for one of them. They will become so accustomed to weighing vices and choosing an evil that they will forget what it means to choose a good. Maybe they already have.
This is why, even though we are not dealing exclusively with the Christian point of view here, we would do well to hear the words of the Christian writer, C.S. Lewis, who explained how one ought to face a choice between two evils:
“I feel a strong desire to tell you – and I expect you feel a strong desire to tell me – which of these two errors is the worse. That is the devil getting at us. He always sends errors into the world in pairs – pairs of opposites. And he always encourages us to spend a lot of time thinking which is the worse. You see why, of course? He relies upon your extra dislike of the one error to draw you gradually into the opposite one. But do not let us be fooled. We have to keep our eyes on the goal and go straight through between both errors. We have no other concern than that with either of them.”
 Mere Christianity, p. 186.