Let's not make this a debate about pro-life and pro-choice views of abortion. According to Roman Catholic teaching, would it be a sin if I voted for a pro-choice politician such as Bernie Sanders?
Yes, if the candidate was voted for because they were pro-abortion
Pope Benedict XVI (formerly Cardinal Joseph Ratzinger) succinctly summed up the Catholic Church's teachings in the memorandum "Worthiness to Receive Holy Communion: General Principles":
A Catholic would be guilty of formal cooperation in evil, and so unworthy to present himself for Holy Communion [due to being in a state of mortal sin], if he were to deliberately vote for a candidate precisely because of the candidate’s permissive stand on abortion and/or euthanasia. When a Catholic does not share a candidate’s stand in favour of abortion and/or euthanasia, but votes for that candidate for other reasons, it is considered remote material cooperation, which can be permitted in the presence of proportionate reasons.
In other words:
- Voting for a pro-abortion candidate because they are pro-abortion is a mortal sin
- Voting for a pro-abortion candidate despite you not supporting that stance is remote material cooperation*, but may be permitted (i.e. not sinful or a venial sin) if you have proportionally strong reasons for doing so
* "Remote mediate material cooperation is cooperation by supplying the occasion or material for the commission of a morally wrong action, but is further removed from the immoral act itself. In the nursing home example above, the person who sells drugs to the nursing home would know the kind and quantity of drugs sold. She has no intention of providing the means for the nursing home to endanger patients by drugging them into a stupor to reduce staff requirements. She sells drugs to legitimate customers and provides any informational assistance necessary about the drugs. Her intention may be to earn a living in a field that can heal or reduce the suffering of others. Remoteness can vary by degrees, with one cooperative action being more remote than another." (Source)
So are Catholics obligated to vote for a pro-life candidate, regardless of their other positions?
Based on the above, a Catholic is not obligated to vote for a pro-life candidate if they have good reason not to, although that good reason should be weighed in proportion to abortion. So being pro-life does not automatically give the politician a free pass over any other moral failings or guarantee a Catholic vote.
To use an extreme example, say that a politician were against all forms of abortion, but planned to commit genocide against every minority ethnic group in their country. In that case, a Catholic would not be obligated to vote for them simply because they are pro-life, and would be permitted to vote for a pro-abortion candidate who does not advocate for such atrocities. On the other end of the spectrum, if getting a tax break is your sole reason for voting for a pro-abortion candidate, it's unlikely to be considered a proportionate reason when compared to the killing of an unborn child, and the Church may consider that sinful.
What about voting or campaigning for a pro-abortion ballot initiative?
Voting specifically for a pro-abortion law on a ballot would be sinful, as would campaigning for such a position. From the same memorandum, which cites the encyclical Evangelium vitae:
In the case of an intrinsically unjust law, such as a law permitting abortion or euthanasia, it is therefore never licit to obey it, or to 'take part in a propaganda campaign in favour of such a law or vote for it' [...] This cooperation can never be justified either by invoking respect for the freedom of others or by appealing to the fact that civil law permits it or requires it
Catholicism does not keep a comprehensive list of what does and does not constitute a sin; there are too many actions with moral consequence to allow for such a list. Although the Catechism of the Catholic Church does point out several actions (including procuring an abortion) as sins, even grave sins, it says nothing specifically about voting for candidates who are in favor of continued legal access to abortion.
The U.S. Conference of Catholic Bishops, however, has published a document titled "Forming Consciences for Faithful Citizenship" (available as a PDF online) which discusses this among other matters. "Forming Consciences" has this to say about voting for such politicians:
Catholics often face difficult choices about how to vote. This is why it is so important to vote according to a well-formed conscience that perceives the proper relationship among moral goods. A Catholic cannot vote for a candidate who takes a position in favor of an intrinsic evil, such as abortion or racism, if the voter’s intent is to support that position. In such cases a Catholic would be guilty of formal cooperation in grave evil. At the same time, a voter should not use a candidate’s opposition to an intrinsic evil to justify indifference or inattentiveness to other important moral issues involving human life and dignity.
So, what is "formal cooperation in grave evil"? This has to do with the way that the Catholic Church has analyzed actions in regard to their sinfulness. Pope St. John Paul II described it in his 1995 encyclical Evangelium Vitae (The Gospel of Life):
Such cooperation occurs when an action, either by its very nature or by the form it takes in a concrete situation, can be defined as a direct participation in an act against innocent human life or a sharing in the immoral intention of the person committing it.
In other words, even if an action doesn't specifically accomplish something evil, if it expresses an agreement with the intent of such an action, it's still evil. For example, the driver of a getaway car isn't, technically, doing anything wrong—just driving a car. But because she's driving the car in support of a bank robbery (which is itself evil), her action is evil as well—it's formal cooperation with the bank robbery.
Thus, the U.S. bishops' position is that voting for a pro-choice politician because the voter is in favor of abortion is support for the grave evil of abortion, and is therefore evil—a sin—itself. However, "Forming Consciences" continues:
There may be times when a Catholic who rejects a candidate’s unacceptable position may decide to vote for that candidate for other morally grave reasons. Voting in this way would be permissible only for truly grave moral reasons, not to advance narrow interests or partisan preferences or to ignore a fundamental moral evil.
(paragraph 35; emphasis added)
It appears, then, that the generally accepted position of the United States bishops is that voting for a pro-choice politician is not in itself a sin, unless the intent is to support abortion. However, one ought not to do so except to advance some other truly important moral issue. What such issues are, indeed whether they exist or not, is not addressed by the bishops; many Catholics (including many of the bishops) sincerely believe that there are no more important issues.
The question is very well addressed in a guide published by Catholic Answers.
In a nutshell, the answer is that a Catholic (and, frankly, every human being, regardless of religion) is bound in conscience never to support public policies that encourage abortion, and this duty includes the grave obligation to vote for those candidates who will best protect the life of unborn children.
There are cases in which casting a vote for a pro-abortion politician is justifiable, although in those cases, one must be careful never to appear to support or promote that candidate’s pro-abortion stance.
For example, suppose that all of the viable candidates for president in a given election cycle are pro-abortion. Suppose that, in the judgment of the voter, since candidate X belongs to a political party that by and large has been in favor of policies to protect unborn children, it would probably be better to vote for candidate X over candidate Y. Although neither candidate is really serious about protecting the unborn, in the voter’s judgment, at least candidate X will do less damage to them.
In that case, the voter is not so much casting a vote for candidate X as against candidate Y, who would likely be worse; and such a vote, made for those motivations, would be morally licit.
Or else, suppose that there are two viable candidates: candidate X professes to be pro-life (as regards the unborn, anyway), and candidate Y is pro-abortion. However, candidate X is also pro-euthanasia, whereas candidate Y is not. These are issues of a similar moral gravity (both unborn children and the elderly and infirm are innocent persons placed at grave risk of death). The voter might reason that, given the current moral climate—since abortion is already legal—it is more urgent to eliminate the threat of a legalization of euthanasia, and hence casts his vote for candidate Y.
Note that the Church teaches that direct abortion is gravely wrong in all cases, even when the pregrancy results from rape or incest. (Procedures whose aim is to save the mother’s life, yet unintentionally result in the death of the child, are generally licit, provided the danger to the mother is present and grave.) As Pope St. John Paul II states in Evangelium vitae,
Therefore, by the authority which Christ conferred upon Peter and his Successors, in communion with the Bishops … I declare that direct abortion, that is, abortion willed as an end or as a means, always constitutes a grave moral disorder, since it is the deliberate killing of an innocent human being (no. 62).
This teaching often poses a dilemma to Catholic voters (and to people of any religion who are seeking to follow the natural law): many candidates profess to be pro-life, and yet admit of exceptions in the case of rape or incest. In a case like this, the same reasoning as above applies: if there are no viable candidates who pledge to protect all the unborn—and especially if the other candidates are fully pro-abortion—then a Catholic may cast his vote for a candidate who is pro-life-with-exceptions.
In sum, every Catholic (and, actually, every human being) is bound to seek an end to the practice of abortion. When voting, he is bound to consider the abolition of abortion as a non-negotiable issue. Ordinarily, he should never vote for a pro-abortion candidate. However, cases may arise (and often do arise) in which there are no viable pro-life (or fully pro-life) candidates. In that case, voters may vote in such a way as to limit the evil committed as best they can.