Catholics who intend to vote for Trump in the upcoming election often give one of three reasons for their decision. They intend to vote for a pro-life politician. They intend to stop Clinton from being elected. They intend to communicate their lack of support for the Democratic party. However, none of these is morally compelling; someone who agrees that these reasons are valid may still decide that a vote for Trump is unjustified. In fact, the claim that any of these reasons gives a moral obligation to vote for Trump reveals an implicit acceptance of the secular ethical system of consequentialism.
Voting for a Pro-Life Politician
Suppose someone is considering this reason to vote for Trump. Because a consequentialist believes she has an obligation to choose the action with the best consequences, she would compare the foreseeable consequences of Clinton's election and the foreseeable consequences of Trump's election, decide which is a better state of affairs overall, and acknowledge an obligation to vote for the "lesser of two evils." If she believes that abortion is wrong and that Trump is more likely to limit the overall number of abortions than Clinton is, she may conclude that her obligation is to vote for Trump.
However, Catholic moral reasoning is not restricted to simply comparing outcomes. In fact, it acknowledges the existence of inherently evil acts, such as lying or the intentional killing of innocents. Even if a lie, murdering, or torture would have good consequences, it is not a moral act and should not be chosen; a consequentialist has no theoretical resources to avoid these kinds of actions. Consequences still play a role in assessing whether or not we ought to perform an action, but as part of the total circumstances which one ought to take into account (see 1749-1761 of the Catechism). What one intends to bring about by one's action is also considered.
How would someone using Catholic moral theory assess this reason to vote for Trump? Rather than merely comparing the outcome of a Trump presidency and that of a Clinton presidency, he will compare the positive and negative aspects of a Trump presidency on its own, examining what Trump has said both about abortion and about other issues. If Trump is against abortion, other circumstances may rule out voting for him: for instance, if one gives credit to the claims that he is in favor of torture and ambivalent about nuclear attacks on other nations — even if one foresees that a Clinton administration would be worse. On the other hand, if one believes Trump to be a pro-life but inexperienced and harmless politician, then the circumstances may justify voting for him. This applies to arguments that Trump is likely to nominate pro-life Supreme Court Justices, etc.
Preventing a Clinton Presidency
A consequentialist with the goal of preventing a Clinton presidency will argue that a vote for a third party or a refusal to vote is morally equivalent to a vote for Clinton, because it has the same consequences: a state of affairs in which Clinton is slightly more likely to defeat Trump. This is because the consequentialist cannot distinguish between causing and allowing things to happen.
This can be contrasted with the well-known Catholic principle of double effect: when a good or neutral kind of action has two effects, one intended and good and the other foreseen and bad, and the good effect is proportionately greater than the bad effect, it can be justified. For instance, the Church has stated that a physician can give a patient painkillers intending to dull the patient's pain, even if this may hasten the patient's death somewhat; giving painkillers in order to bring about the patient's death, however, would be the different (and intrinsically wrong) action of euthanasia, even if both actions appear identical to an observer. The consequentialist, on the other hand, cannot make a moral distinction between these actions because they each have the same consequences.
Can someone with a Catholic moral perspective vote for Trump in order to prevent Clinton from being elected? Definitely, if electing Trump can be justified. Aquinas specifically stated the conditions of the principle of double effect while arguing that it is permissible to kill someone in self-defense (see the Summa Theologica, II-II, Qu. 64, Art 7). Defense of one's country seems analogous to self-defense: one is attempting to preserve a very great good. However, just as there are things that we cannot do in self-defense (for instance, killing an innocent person rather than an aggressor), there are things that one cannot do in preserving the US from a Clinton presidency.
In order to apply the principle of double effect here, one must examine the intended effect, the foreseen effect, and whether the evil permitted is proportionate to the good that would be brought about. Once again, one needs to consider the type of person Trump is, his promises, and what one foresees him legislating. If the good which would be destroyed by a Clinton presidency significantly outweighs the negative aspects of a Trump presidency, voting for Trump can be justified. If, on the other hand, the good which would be destroyed by a Clinton presidency is not significant, voting for Trump may not be justified.
Communicating Disagreement with the Democratic Party
On the whole, this seems to be the weakest argument to vote for Trump. A pro-life president would be a great good which could justify voting for a candidate even if he is bad in other respects. One's country can also be a great good, and we can justify bringing about a certain amount of evil in order to defend it. However, communicating one's affiliation seems to be a much less significant good than a pro-life president or one's country. A lesser good, like this one, can justify a much smaller proportion of evil. A Catholic approaching a vote for Trump for this reason must sincerely ask himself whether the negative aspects of Trump's campaign are proportionate to the good that would be effected in bringing his disagreement to the Democrat Party's attention, even if he doesn't believe that Trump will be elected.
Moreover, this goal seems to give a reason to vote third party rather than Republican; after all, this would communicate to both parties that one is unsatisfied.
While the consequentialist has the ability to simply compare possible outcomes in order to assess what action to take, a Catholic moral theorist must look more closely at the actions themselves. In contemporary political discussions, those arguing that every Catholic has an obvious duty to vote for Trump are implicitly affirming a consequentialist ethics. From the perspective of Catholic moral theory, each person needs to research and honestly assess whether what they foresee Trump doing is a circumstance that justifies voting for him. As the bishops have stated, we need to "[focus] more on moral principles than on the latest polls."