In a well argued facebook post, Patrick O'Donnell maintains that a vote for a third party candidate is morally irresponsible. Here is what he has to say:
The upcoming election is a binary choice (i.e., abstention or third party voting is morally and politically irresponsible), and here's my brief argument as to why:
If one assumes (and it seems a reasonable assumption given what current numbers suggest and 'experts' are saying about the upcoming presidential election) that this presidential race will be fairly close and, furthermore, believes that Donald Trump is an immeasurably worse candidate than Hilary Clinton, than it would be politically (and I think morally) irresponsible to vote for a third party candidate of any sort, for that would be tantamount to giving your vote to Trump. If one believes a vote for a third party candidate amounts to a salutary or necessary expression of one's ideals or acting according to one's principles and is therefore justified in voting for a third party candidate under these conditions, one is being an idealist in the worst (or at least a pejorative) sense, the sort that ignores the real world consequences of one's actions (in this case, simply the act of voting).
Even principled idealists take into consideration what is possible or probable when acting on their principles, and do not proceed come hell or high water (as we say), blissfully ignoring the (possible or, in this instance, likely) results of their actions. Voting "on principle" or according to one's ideals sans consequentialist considerations may be seen as an act of moral "purity" in some sense (the attempt to 'wash one's hands' of this moral mess) when examined solely from the vantage point of the individual, but one's actions are inevitably and inextricably bound up with the actions of others and contribute to the outcome of those actions. There are myriad other _and_ more morally and politically responsible ways to directly realize or act on one's ideals and principles: indeed, to vote for a third party candidate may, over time, serve to thwart the realization of one's most cherished principles, values, and ideals.
To vote for a third party candidate under the aforementioned conditions amounts to the vice of "moralism" (what this means is fairly complicated, but for those interested, please see C.A.J. Coady's brilliant little book, Messy Morality: The Challenge of Politics, 2008) which includes acting without "a breadth of understanding of others and of the situations in which she and they find themselves. In addition, or in consequence the moralizer is subject to an often-delusional sense of moral superiority over those coming under his or her judgment." Lest I be misunderstood, I want to make it clear that I'm not saying that the rational dictates of prudence or consequentialist reckoning in this case trumps acting on moral concerns. Rather, and again in the words of Coady, "Morality should certainly be attentive to circumstances and the way it conditions what is possible." So, and by way of a conclusion, we are faced in fact with a binary choice if we are to act in a morally and politically responsible manner, in other words, abstention or voting for a third party candidate is succumbing to the vice of moralism and feigning ignorance of the morally significant consequences of such a choice.