"Between two evils, I always pick the one I never tried before." - Mae West
Because both Clinton and Trump are quite unpopular, even within their own parties, many Republican and Democratic voters will face an unpalatable choice in November. When local, state, and other national races are considered, similarly unpalatable choices are not rare, so it will be useful to investigate what voters should do in general terms.
If you are unhappy with both the Republican and Democratic candidate in a swing state or close election how should you vote?
(a) Vote for no one.
(b) Vote for the best candidate, regardless of whether he or she is viable (i.e. has a reasonable chance of winning).
(c) Vote for the worst viable candidate (the Greater Evil).
(d) Vote for the best viable candidate (the Lesser Evil).
Many articles have recently appeared urging voters to do one of these things. Typically, these articles assume that the choice is obvious, and that dissenters are therefore foolish or acting in bad faith.
I don't think that the right answer is obvious. But there is a right answer: vote for the Lesser Evil. 
- As Aristotle says, decision-makers in a state should aim at the common good. 
- Therefore, your goal should be to elect the best possible candidates.
- A general principle of action: When the ideal outcome in a situation is unachievable, you should opt for the best possible alternative.
- Therefore, you should vote for the best viable candidate.
- Therefore, if neither of the viable candidates is a good candidate, you should vote for the Lesser Evil.
Objections and Rebuttals
(A) Objection: The best alternative for the USA at the moment is not the Lesser Evil. Instead, the USA would be better off ENDURING THE HARM caused by the victory of the Greater Evil in order to make a certain sort of social progress in the long run. (For example, the shock might spur the USA to make big, positive social changes, or time in the wilderness might reform the losing party, or increasingly disaffected voters might build a viable third party). Therefore you should vote for the best candidate even though he/she has no reasonable chance of winning.
Rebuttal: The victory of the Greater Evil would be best for the USA only if (a) it would have a reasonable likelihood of producing the desired effect (e.g. the shock, change, reform, or third party), (b) the projected harm inflicted would not be worse than the benefit, and (c) the harm would not fall overwhelmingly upon already-disadvantaged groups. But these conditions are almost never fulfilled.
(B) Objection: If people who are regularly unhappy with the viable candidates vote for the Lesser Evil in election after election, their votes will be TAKEN FOR GRANTED by the establishment. That will be bad because, although major changes to the way things work are necessary, they will not be made. Therefore, at some point people who seek major changes must refuse to vote for the Lesser Evil, even though doing so may allow the Greater Evil to win, and thus harm the USA in the short run.
Rebuttal: There are many other, less harmful ways for people seeking major changes to achieve their goals. For example, they might organize demonstrations and letter-writing campaigns, give speeches and publish essays, etc. That is, they might raise consciousness, persuade voters, and pressure the establishment to make major changes. Alternatively, they could start building a campaign now, and get viable candidates favoring major changes onto the ballot in 2020.
(C) Objection: The best way to IMPLEMENT GOOD POLICIES is for voters to vote for the best candidate regardless of whether that candidate is viable. For even if the best candidate is not elected, each vote for that candidate endows his/her policies with more influence.
Rebuttal: This approach makes sense in a multi-party system where candidates with small percentages of the vote nevertheless may exert influence through coalition governing. But in the USA's two-party system, candidates with small percentages of the vote exert no influence at all.
(D) Objection: Even though each side in our adversarial judicial system strives to forward its own interests rather than aiming at justice, the structural features of the system make it a reasonably good procedure for achieving justice. Similarly, elections have STRUCTURAL FEATURES which generally yield the best candidate when each voter disregards viability, and votes for the candidate he or she considered best. Thus, you should vote for the candidate whom you consider best, regardless of whether that candidate is viable.
Rebuttal: There is no reason to think that elections are such a system. When many voters ignore viability, a common result is that several non-viable candidates split the vote in such a way that the Greater Evil wins.
(E) Objection: The voter's duty is to vote according to his or her CONSCIENCE, even if that is counter to the common good because one is never certain about what will happen. Therefore, you should simply disregard expected consequences and avoid wrongdoing by voting for the best candidate, or for no one.
Rebuttal #1: Voting one's conscience sounds noble, but it is actually quite counterintuitive. Nothing in life is certain, but much is reasonably predictable, and reasonable people base their choices on these predictions. When a child falls into a stream on posted private property, you don't say, "Who knows whether the child will drown or survive, but I know trespassing on private property is wrong, so I will avoid wrongdoing and hope." Instead, you jump in to rescue the child. Similarly, you shouldn't say, "Who knows what will happen to the country. I'll just vote for the best candidate and hope." Instead, you should base your choice on your best guess about what will happen, and that means voting for the best viable candidate, the Lesser Evil.
Rebuttal #2: This objection is based upon a confusion about duty. Your conscience shouldn't say, "Trespassing is always wrong." Instead, it should say, "Trespassing is usually wrong, but it is the right thing to do when necessary to save a life." Similarly, your conscience should say, "Voting for a candidate whom you expect to do wrong is generally bad, but it is the right thing to do when necessary to prevent even worse wrongdoing." Thus, if your duty is to vote according to your conscience, you should vote for the Lesser Evil.
(F) Objection: If you vote for candidates who are likely to do wrong, you become complicit in that candidate's wrongdoing. In order to AVOID RESPONSIBILITY for wrongdoing you should vote for the best candidate or for no one, even if that is counter to the common good.
Rebuttal: This is a choice to keep your own hands clean at the expense of the common good. You are choosing to let others suffer so that you can feel good about yourself. Not only is this a selfish choice, it will fail to achieve its goal. If you want to keep your conscience clear, you will need to get your hands dirty.
(G) Objection: The voter's duty is to MAKE A STATEMENT, even if that is counter to the common good.
Rebuttal: But what is the point of the statement? If the point is to persuade other people, and thus make social progress, then this is just a version of objection (C). If the point is not to persuade anyone, but merely to register one's stand, then this is just a version of objection (F).
Consider a small-scale voting situation. Suppose your hiring committee has narrowed the applicants for a job to a short list of two candidates, Dick and Jane. You think both Dick and Jane are seriously flawed candidates, but Dick is clearly better than Jane. You also think that Sally would be a great hire, but Sally did not make the short list. Should you vote for Dick, Jane, Sally, or abstain?
(a) You could abstain, even though your abstention will not prevent the hire.
(b) You could vote for Sally, since Sally is the best candidate, even though Sally is not a live option at this point in the hiring process.
(c) You could vote for Jane, trying to shock the committee into its senses.
(d) But you wouldn't do any of these things. Instead, you would vote for Dick, the Lesser Evil. 
 The strength of the duty depends on the differential between the two evils, and on the absolute size of the evils." (Sophie Grace Chappell, in conversation)
 The common good is a tricky, technical notion. It does NOT mean, "whatever is good for the nation as a whole, even if that good is achieved through exploitation of the individuals." Nor does it mean, "whatever is good for every single individual." No policy meets that stringent condition. Every policy has both losers as well as winners. So the common good means roughly, "whatever is good for the bulk of the individuals, harms only a few people, only mildly, and not the already worst-off group."
 I am indebted to R. Barney, A. Epstein, and S. Rushing for very helpful input. It should not be assumed that they agree with any of my arguments.