When we say "voting is a moral act," we have to define our terms. The "moral act" means different things depending on what philosophy you espouse and how you define morals. Notice that "morality" is not the same as "ethics." Morality is a divinely revealed code of conduct. Ethics is a humanly agreed upon code of conduct. There is overlap between the two, but the two are not identical. I can do something that is moral, but unethical, and I can do something that is ethical, but immoral.
Take the example of the doctor who sleeps with his patient. Immoral? Unethical? Both? Neither? We can't tell from just the example. If the doctor's patient is his wife, then the act is perfectly moral, but it might be unethical if his particular specialty (for example, surgery) requires that doctors not accept close relatives as patients. On the other hand, if the patient is NOT his wife, the act is certainly immoral, but might be ethical because his particular specialty (for example, podiatry) lays no specific prohibitions on doctors having emotional relationships with patients.
So, if we accept that morality is a divinely revealed code of conduct, then the moral act, at least in Christian tradition, is made up of three elements: circumstances, intention and the act itself. Notice that the outcome is most definitely NOT part of the moral act. Again, in Christian tradition, God is the one who brings about the outcome, not man. Man proposes, God disposes. We do our best, but our best is not expected to be good enough to always accomplish our intention.
And so it is with voting. When we vote, we are not responsible for the outcome. Whether the person for whom we vote wins or loses is not our concern. We vote (act) for the candidates who fulfill the legal requirements of office (circumstance) with the intention of placing them in office. Whether they actually make it into the office, or even have a chance to make it into office, is not relevant to the morality of the act, which is my vote. As long as I vote with the intention that my vote helps this person get into office, I have provided the third element of the moral act.
In order for a vote to be a morally good act, all three elements have to be good, or at least morally neutral. For example, I have no right to intend to put an unqualified person into office. An unqualified person is not just someone who doesn't satisfy the laws of the state concerning the office. After all, a person may fit the secular qualifications (correct age, mental capacity, etc.), but still be unfit because that person is known to be morally unqualified. Perhaps he abuses power, perhaps he regularly and without remorse uses power to injure and/or kill others. Such a person is not worthy of my vote.
Now, I am aware that my vote is one out of hundreds of millions cast. There is simply no reasonable probability that my vote alone will actually put anyone into office or prevent anyone from taking office. Given the number of votes cast, my vote has essentially zero effect on the outcome of any election. In fact, as we have seen, even if it were otherwise, even if I were reasonably certain that my vote would accomplish my intention, the placement of a specific person into a specific office as a result of my vote would be an outcome, and outcome isn't part of the moral equation.
So, when I vote, I don't vote in order THAT someone may win. Rather, I vote in order to express the idea that this person is someone I know well enough and I trust well enough to act correctly while they are in office (whether they get that office or not). My vote is a statement about how much I trust another person, a statement that asserts the office-seeker's values are close enough to my own that I have good reason to believe he will serve others well while in office.
It is only in THAT sense that my vote is a moral act. My vote is a short-hand letter of reference. I don't look at the office-seeker and say "Well, he's not as bad as the others, and someone has to do the job, so I suppose he will have to do." Rather, I look at the office seeker on his own, without reference to the other office-seekers. Based on the assessment I make of this seeker alone, I determine if I know him well enough and trust him well enough to endow him with the power of the office.
This is the important part: If none of the office seekers are trustworthy enough, then none of them get my vote.
If voting is a moral act, then my vote is my personal moral statement.
That is all it is - it can be no more.
But neither can it be any less.
As a Catholic, I have a duty to make publicly clear that I am Catholic. If I can do that by voting for a particular candidate, thereby declaring that this candidate's values match true, substantial Catholic values, then I am bound to vote for the candidate. I cannot not vote for him. However, if I cannot find a candidate whose values are substantially Catholic, if I see all the candidates have values that substantially violate Catholic faith, then I am duty-bound not to vote, I am duty-bound NOT to endorse them with my vote. I am duty-bound NOT to contribute to handing them the power of office.
I do not have to choose between Lenin and Stalin, between Pol Pot and Mao tse Tung, between Goehring and Hitler. If these are the choices, then the Catholic thing to do is to imitate St. Thomas More, retire to my estate and refuse to make any public endorsement of anyone.
If I predicate my vote on what the outcome will be, I am playing God. I have, at that moment, become a consequentialist, a modernist, a fool. I am representing to myself that I have control over outcomes that I do not, in fact, have any control over at all. By the very fact that I already know my vote will not, and never can, be the single deciding vote in an election involving 330 million people, then pretending my vote is necessary for a specific outcome is farce. It is just me trying to convince myself that a dreamworld is reality, that my vote means more than it actually does. I represent myself as having more power than I could ever, even in the wildest reality, actually wield.
The choice in 2016 is between Lenin and Stalin, between Pol Pot and Mao tse Tung, between Goehring and Hitler. Thus, for the time being, I retire.