Evil is real. There is an objective moral order. This post cannot speak to moral relativists, or to people who define morality by what they personally like or dislike. For those who seek objective moral standards, the Catholic Church has provided the most philosophically sound ones available. Here are some standards of cooperation in good and evil and the mechanism of voting.
We are speaking here of moral evil, of a human being who chooses to reject some aspect of the goodness of God's creation, for corrupt, unnatural, reasons. People can choose to be terrorists. They can choose to be racists. They can choose to be abortionists. They can choose to be pornographers. In addition to these evils, people can perpetuate others evils by actions that cooperate in the evil.
If a cable company carries internet sex, they are cooperating in that evil activity, even though their goal is only to succeed in business. The same is true of the gun dealer who sells weapons to people of evil intent. The same is true of a medical company that makes equipment used for abortions. The company does not perform abortions, but their business cooperates with the business of abortion.
These examples are simple transactions. But broader examples get more complex so that it is not easy. The government promotes things that are evil. When a citizen pays taxes they support the government. Is this cooperation in evil? What about voting? If I cast my vote for a candidate, and they commit evil as an elected official, haven't I cooperated in that evil because I helped elect him or her? The Church makes a distinction here, and I will try to apply it. Many more complexities can crop up, but we will simplify the matter for the sake of this introduction.
No one choice, or dozen choices, can eliminate all evil. When we intend to do good, and even accomplish some good aim, sometimes we keep a chain of causality moving, which at the same time, allows evil. The Church calls cooperation in evil, which is not intended specifically to perpetuate that evil, Material Cooperation. If I am the UPS man, and I deliver loads of fertilizer, I may be cooperating in the evil act of making a bomb. If I pay my taxes, I may be cooperating in some government program which promotes sex ed materials in school contrary to God's natural moral order. And, for the voting example, if I vote for a candidate who seeks to institutionalize racism, I may be cooperating in the formation of unjust policies.
Here’s the distinction. The Church's morality allows that we may cooperate materially in evil, without the guilt of sin, if it is "Remote"; that might be explained, if the evil is so far removed from us that there is really nothing we can do to change it. When the evil is "Proximate," or, when it is within our reach to do something about it, then we must do something to try to eliminate this evil, or stop whatever activity we do which cooperates in that evil. I may not be able to stop paying taxes, claiming the unrealistic goal that it will stop immoral government programs. I may be able to cancel some service or business transaction, because I see that it will stop or slow the evil committed by a business provider. I may work in a hospital that performs abortions. But if the only thing about my job that endorses or facilitates those abortions is that it contributes to the overall operation of the hospital, then my cooperation in that evil is remote, and I have no moral obligation to quit my employment at that hospital.
In order to make the full comparison to voting, we need the term that contrasts with Material Cooperation, that is, Formal Cooperation. When a person cooperates formally in an evil act, they are not performing the evil act, but they are endorsing that evil act, because their action specifically facilitates that act. Politicians can vote to enter an unjust war. That is formal cooperation in evil. They may vote to eradicate the natural order of marriage by so called "marriage equality" policies. That is formal cooperation in evil. They may seek to promote abortion by executive acts. That is formal cooperation in evil. The appointed or elected officials here do not perform the actual acts of war, or unnatural marriages, or abortions, but they endorse them in a positive way. This is formal cooperation. Formal cooperation is evil upon evil. It is in fact a second evil added to the evil thing cooperated with.
Now, here is the fine point. It is not necessarily formal cooperation in evil to vote for a candidate who cooperates formally in evil. Under the circumstances of a formed conscience, the morality of the Church “allows” voters to cooperate materially in evil by voting for candidates who cooperate, formally, in evil. Yes. It is indeed "cooperation in evil." But - and this is key - if a voter does not intend to endorse those evils, but only intends to endorse the greater good that they think this candidate will promote, as opposed to any other "electable" candidate, then the Church considers this cooperation "remote" enough that it is allowable.
There are two things here to note. Firstly, if a voter votes for a candidate because they think some of his actions are good, which the Church has called evil, then the Church judges this vote akin to more formal cooperation. The voter intends to promote the evil because they think it good. They are a heretic endorsing immorality. Maybe they do not know the objective morality involved because they are a lapsed Catholic or they are completely pagan. Ignorance of morality and ignorance of Christ are topics for another article. Here the topic is about those people who could and should know some objective standards for moral acts. For our purposes, if I vote for a pro-racism, pro-abortion, anti-family, anti-poor, or any otherwise evil intending candidate, because I intend to further his evil intentions in these areas, against the morality of the Church, I am doing evil myself. I am formally cooperating by endorsing his formal cooperation.
Secondly, it must be granted that a vote for any candidate will likely entail some sort of cooperation in evil. We must note that the Church has not imposed any exact scale by which voters must calculate which candidate has a higher good-verses-evil cooperation score. The first point above, as serious as it is, has not led the Church to make any pronouncement, "You may not ever vote for a candidate that supports X, or that is against Y." Many good Catholics will hold to principles such as these, with a validity of personal conscience, but no pope or bishop has made such definitive and binding pronouncements for all believers who enjoy suffrage in their country. Similarly the Church has avoided so far giving an exact "score" of what is the "best" candidate in any particular election based on the Catholic morality system.
This is the current state of things, but it also is as detailed as we can get with the moral principles of the cooperation in evil. It is very frustrating for everyone. I, as a priest in the United States, must almost say, "let us prepare for the persecutions." For, I can give the Church's morality and direction, but it is certain to satisfy no one. As a Catholic United States citizen I can say both "If pro-abortion, anti-family Hillary Clinton is the best choice we have for President, then we are in very dire and desperate times," and "if pro-greed, anti-solidarity, anti-subsidiarity Donald Trump is the best choice we have for president, then we are in very dire and desperate times." The political strategy, whereby Donald Trump might be more favorable to a Christian social strategy, certainly seems like a challenge and an opportunity for the Church. But some Catholic laity, who dare to take the role of a prophet, are already saying "persecutions and evils suffered under Clinton would be better than the material cooperation in the evil of Trump." This is the argument among many faithful Catholics. "If one will take us to hell in a hand basket, and the other will just take us there slower, do you really want the Church to endorse either one? Are we not in the exceptional circumstances where Catholics might abstain from voting for "electable" candidates in any particular race?" In the mean time, the Church still has not endorsed any candidates.
We may end with the nostalgic reminiscence, that when the Apostles first set forth to preach, they were in a world where no one was called upon to exercise "faithful citizenship" with the thought that they could influence the political sphere of the world's powers. It is both our problem and our opportunity that a citizen's vote in our country may in fact accomplish something, for good or for evil. But in both the ancient world and now, the Church's response to the cry "we are going to hell in a hand-basket" was never, and will never be, "go and vote in this way or that." The Church's response is "go show yourself to the priest," "repent and believe in the Gospel," "for God has made him both Lord and Christ, this Jesus whom you crucified."