The purpose of this post is to answer the question: "Is it rational to vote for third party candidates?" Let me first define the words I am using. By "rational" I simply mean choosing the wisely among alternatives. A rational person weighs the various options (calculates ratios) and chooses the option the he/she thinks will do the most good. A rational person aligns their actions with their goals. By "third-party candidate" I am simply referring to a candidate that is not one of the major two parties in the United States—namely Democrat and Republican.
I want to go through a series of thought experiments to try to answer the question of this post.
Let's say that Barack Obama and Mitt Romney are the only 2 candidates on the ballot. Let's say that between the two candidates, a Romney presidency is 100% aligned with my goals and an Obama presidency is 0% aligned with my goals. (Saying that Romney is 100% aligned with my goals is the same thing as saying that I agree with everything that he says.) In this scenario, it is rational to vote for Romney.
Now let's say that Romney is 1% aligned with my goals and Obama is 2% aligned. Since Obama is more aligned with my goals, it would now be rational to vote for Obama.
Now let's add a third-party candidate. The candidates are Romney, Obama, and Jesus (insert perfect candidate of your choice here). Let's say Jesus is 100% aligned with my goals, Romney-50, Obama 49. Let's also stipulate that they all have an equal chance of winning. In this case it is rational to vote for Jesus.
Now lets take the same example—Jesus-100, Romney-50, Obama-49. But Jesus happens to have no reasonable chance of winning an election. But, Romney and Obama have fairly equal chances of winning. What is the rational thing to do? As I defined in the beginning "rational" means choosing the best option among alternatives. Because Jesus has no chance of winning, I believe that it is rational to vote for Romney over Jesus. Why? Because although Jesus would do the most good, the good that Jesus would do if he were elected doesn't matter. It is simply not part of reality. A vote for Romney in this case would do more good in the world than a vote for Jesus.
Conclusion If scenario 3 was representative of the situations that we find ourselves in, then it would be rational to vote for a third party candidate. However, in my lifetime there has never been a third party candidate that has had a chance of winning. Every election that I can remember resembles scenario 4 and there is no evidence that we will depart from scenario 4 anytime soon. Therefore, under normal circumstance, I believe it is irrational to vote for third-party candidates, because that decision will not be aligned with the goals of the voter. Voting for the most electable candidate that you agree with is the best way to influence politics in a direction that you one think is best.
The "Vote on Principle" Objection
Some object by saying that one should vote "on principle" or similarly people should vote "their conscience". I am not really sure what this means, but I think it means that everyone should vote for the person with whom they agree the most whether or not they have a chance for winning. If we apply this principle consistently, then it means that since everyone agrees with themselves the most, they should write in their own names and vote for themselves. This is absurd so perhaps the "vote on principle" advocates mean something like vote for whoever you agree with the most that is a running candidate. But this approach assumes that one can do more good if they vote for someone that is a running candidate even if they agree with that candidate less than they agree with themselves. If that really is the assumption, then it only seems logical to apply it more broadly as I have to vote for someone that will do more good by being elected.
In my opinion, I am voting on principle. The principle I am using is rationality. I want to do the most good among the alternatives available. My conscience leads me to try to do the most good possible.
The "Lesser of 2 Evils" Objection
Some object that I am just settling for the "lesser evil". I might agree with this argument if we were voting between Hitler and Pol Pot, but I don't think that picture matches reality. I think that framing the issue this way is categorical. Anyone judging the candidates to be evil presumes the omniscience of God. The decision making process is not categorical, but incremental—meaning that there are costs and benefits that need to be examined and weighed for each candidate. In other words, the choice between candidates is a choice between 'more good or less good' given the trade-offs inherent in the voting process. In other words, it is pointless to argue what should happen in a perfect world. Instead, we should focus on what can be done in the world that we find ourselves in. If we compare our current situation with the perfect world, then everything will seem evil to us. To get a clear picture, we must compare where we are now, with where we have been.
The "Send a message" objection
Some say that you should vote for the candidate you agree with most to send a message. I admit that in some rare circumstances this may be a rational strategy. For instance, let's say that there are only 2 candidates, Romney and Obama. I agree more with Romney, but he has no chance of winning. In that case I would still vote for Romney because sending a message is the best I can do in that situation.
But most of the time when people say they are sending a message, I think they are really just treating elections as an occasion to vent their emotions, rather than as a process to pick someone into whose hands to place the fate of the nation.
The "They are all the same anyway" objection
Some claim that the major party candidates are all the same and therefore they should vote for some third-party. This claim strikes me as very ignorant. Anyone who is aware of the voting patterns between Democrats and Republicans know that they very rarely vote similarly on any piece of legislation. Perhaps more important, the president has the power to select supreme court justices that can serve for several decades. With very few exceptions, the type of judge that a democrat will select is much different than the type of judge that a republican will select. Saying that one does not like either candidate is not equivalent to saying that they are all the same. We must do our homework and select which electable candidate will do the most good.
The "people will wake up" objection
Some argue that we should let the worst candidate win so that "people will wake up". The problem with this argument is that people don't "wake up". The worst candidate ends up electing judges that sit on the bench for decades that make matters even worse over the long run.