If you’ve ever seen the movie My Cousin Vinnie, you probably remember the scene where Bill Gambini and Stan Rothenstein’s characters are being interrogated about the murder of the store clerk, but they still think they were brought in over shop lifting a can of tuna. Stan said he didn’t even know anything happened until afterwards, and the sheriff says he could still be charged with aiding and abetting. Legally, aiding means doing something to support the crime, either before or after it happened, abetting means encouraging the person to commit the crime, and accessory means doing something to protect the person who committed the crime. 
            In Catholic moral theology we have a similar idea. Helping someone to commit a sin can be a sin itself, depending on how much help you give and whether or not you agree with their actions. This is called either Formal or Material Cooperation and Immediate or Mediate Cooperation. All of them happen when you’ve done something to help someone commit a sin.   Formal Cooperation means that you’ve done something to help them and that you approved of their actions or agreed with them. This is always a sin because your intention was to help them commit an evil action or promote evil. It doesn’t matter if it was a little help or a lot of help, your agreement with their actions makes it formal cooperation.
            Material Cooperation is when you’ve done something to help or promote the evil of another person, but that you are opposed to that evil action but supported it for some other reason. If you’re help was necessary for the evil action to take place (you gave them the gun that they used to murder someone) then it’s still not allowed. If the help was further removed it could be allowed if there was a serious enough reason.
            Voting for a politician for office is cooperating in what they say they’ll do once they’re in office, but it’s very rare to agree with a politician 100% of the time. We shouldn’t vote for someone who promotes or intends to support gravely sinful actions, especially those that threaten the right to life. Pope St. John Paul II wrote about this in Christifideles Laici, “Above all, the common outcry, which is justly made on behalf of human rights—for example, the right to health, to home, to work, to family, to culture—is false and illusory if the right to life, the most basic and fundamental right and the condition for all other personal rights, is not defended with maximum determination.” 
            It may be possible to support a candidate who promotes gravely evil actions, provided that you completely oppose those actions, in two cases. First, if all of the candidates support gravely evil actions, such as if all candidates support abortion, then we must weigh the other issues or, in very serious cases, refuse to vote for any of them. Second, a Catholic can only vote for a candidate who supports gravely evil actions if there is a proportionate reason to do so. 
            When you’re choosing who or what to vote for, remember that you are connecting yourself to that person or issue. I would encourage you to look at all of the option, try to ignore political propaganda, look at what they say and what they’ve done, and always remember to bring it to prayer. We must all ask the Holy Spirit to give us the gifts of wisdom, to know God’s will, and prudence, to know how best to do it.