Does voting for a pro-abortion candidate constitute formal cooperation in his crime of promoting child killing?

St. Alphonsus of Liguori, discussing "What is lawful for servants? And what might be material and formal cooperation?" in Theologia moralis 2 §63, writes (my emphases):

[Flannery transl. p. 27:]
But it is better, with [yet] others, to say that that cooperation is formal which contributes to (concurrit ad) the bad will (malam voluntatem) of the other and cannot be without sin, but that cooperation is material which contributes only to (concurrit tantum ad) the bad action of the other, beside (præter) the intention (intentionem) of the cooperator.

[Grant transl.:]
Rather, it must be said better with others that it [cooperatio] is formal which agrees with the evil will of another, and cannot be without sin; but the material that which agrees only to the bad action of another, apart from the intention of cooperation.

[🇻🇦 original, pp. 253-4:]
Sed melius cum aliis dicendum, illam esse formalem quæ concurrit ad malam voluntatem alterius, et nequit esse sine peccato; materialem vero illam, quæ concurrit tantum ad malam actionem alterius, præter intentionem cooperantis.

Notice, St. Alphonsus doesn't define formal cooperation in terms of whether "one interiorly wants the evil action to be performed" by the malefactor (pace Fr. Hardon's definition of formal cooperation), but in terms of its concurrence in the malefactor's bad will (voluntatem).

It seems a voter who votes for a politician who supports intrinsic evils (e.g., abortion/child-killing, sodomy, bodily mutilation, etc.) "contributes to the bad will" of the politician by investing him with political power, inflating his ego, and not punishing him for but encouraging him to persist in his evil actions, regardless the voter's intentions (e.g., choosing the lesser evil candidate, etc.).

Does voting for a pro-abortion candidate constitute formal cooperation in his crime of promoting child killing?

First of all, what is ”formal co-operation”?

The deliberate concurrence in another person's usually sinful action. The co-operation is formal and always sinful if, besides giving external help of whatever kind, one interiorly wants the evil action to be performed. Formal co-operation is at least a sin against charity by doing spiritual harm to one's neighbor; frequently it is also a sin against another virtue, especially of justice. - Formal Co-operation

That would depend on the situation at hand.

“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.” — Joseph Cardinal Ratzinger

It is never permissible for a Catholic to vote for a pro-abortion candidate because the candidate is pro-abortion. Such a vote would be formal cooperation in the serious sin of the candidate who, upon being elected, would vote for legislation making possible the taking of innocent human life through procured abortion.

When a Catholic does not share a candidate’s stand in favor 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 strictly defined.

Since abortion and euthanasia have been defined by the Church as the most serious sins prevalent in our society, what kind of reasons could possibly be considered proportionate enough to justify a Catholic voting for a candidate who is known to be pro-abortion? None of the reasons commonly suggested could even begin to be proportionate enough to justify a Catholic voting for such a candidate. Reasons such as the candidate’s position on war, or taxes, or the death penalty, or immigration, or a national health plan, or social security, or aids, or h*m*sexuality, or marriage, or any similar burning societal issues of our time are simply lacking in proportionality.

• On Voting for Pro-Abortion Candidates by Bishop Rene Henry Gracida, DD

Here is what Frs. Benedict Ashley and Kevin O’Rourke, O.P have to say about the distinction between “formal” and “material” co-operation:

Sometimes people cooperate with a person doing evil by approving what the person does, or by willfully and knowingly partaking in the evil action…. This is formal cooperation in an unethical act, and it is always wrong. Conversely, I may cooperate with another person, not because I freely approve or cooperate in the evil action, but because I am coerced into cooperating…. When the duress is present, the cooperation is known as material cooperation, and it may be of two different kinds. If one cooperates in an evil act by performing something that is essential for the performance of the evil action, then it is immediate material cooperation. If one cooperates in an accidental or nonessential manner in the evil action, then it is called mediate material cooperation. - No Cooperation with Evil

When all candidates are pro-abortion I could consider voting for the least pro-abortion candidate.

Keep in mind, and it happens, that some pro-life candidates do nothing to amend such laws when they are voted in!

If you vote, study the track record of all candidates before voting day and be well informed about your decisions. Abortion is just one issue, what about euthanasia, assisted suicide, or other moral issues like LGBTQ or unjust wars? Catholics must be always clearly informed on all issues at stake in order to make a proper moral decision here.

The following articles may be of interest to some:

• But Is It Remote Material Cooperation?

• Moral Cooperation in the Evil of Another

In short, no. Voting for a representative is never a formal cooperation with all of his positions (unless the voter happens to actually support all of those positions in his heart, which is exceedingly rare to the point I am comfortable saying it practically does not happen). One must weigh one candidate against another, and, if one must vote for one candidate because he genuinely believes greater evil will result in voting for the other, then, because he only remotely cooperates with the evil of the less bad candidate, he does not sin. The USCCB releases voter guides (See the summary from 2020) before elections. As you can see, neither major candidate aligns perfectly with Catholic teaching, and no one says that Catholics must vote third party.

A Catholic Answers article goes into detail about the different nuances in this issue. A summary is that you may vote for a pro-abortion candidate if you have proportionate cause to do so, but you may not vote for the pro-abortion candidate in support of their pro-abortion position. This is true of any public policy position a candidate has which contradicts Catholic teaching. In considering proportionality, we should consider that the USCCB has continuously stated that abortion is the pre-eminent moral issue for Catholic voters to consider. Therefore, it should be given the greatest weight in evaluating the candidates. It is a high bar to overcome for the Catholic voter (at least in America) to be truly justified in voting for the pro-abortion candidate, but at least in principle it is possible to be so justified.

In case the gravity of abortion is lost in translation, so to speak, here, we should consider another opinion on the liceity of voting for a pro-abortion candidate. Father Torraco states that abortion is a "disqualifying issue," and that Catholics may not vote for a pro-abortion candidate for this reason. However, question 10 in his FAQ offers support for the assertion that voting for a pro-abortion candidate is not necessarily formal cooperation with evil (since formal cooperation is always a sin). He says

Obviously, one of these candidates [who are all pro-abortion] is going to win the election. Thus, in this dilemma, you should do your best to judge which candidate would do the least moral harm. However, as explained in question 5 above, you should not place a candidate who is pro-capital punishment (and anti-abortion) in the same moral category as a candidate who is pro-abortion. Faced with such a set of candidates, there would be no moral dilemma, and the clear moral obligation would be to vote for the candidate who is pro-capital punishment, not necessarily because he is pro-capital punishment, but because he is anti-abortion.

It is manifestly evident that one does not formally co-operate with the wickedness of abortion in this case, therefore, voting for a pro-abortion candidate is not necessarily a formal cooperation with the evil of abortion.

Now, it is clear that abortion is not the only disqualifying issue. Euthanasia and genocide are also disqualifying issues, in the same camp as abortion. So, given the choice between a number of candidates which support these very grave disqualifying issues, and no candidate who opposes all of them, it is evident from what Father Torraco says that one ought to choose the candidate who "would do the least moral harm."

But keep in mind that very few issues are actually disqualifying. Some would try to have you believe that issues like racial equity, capital punishment (explicitly covered by Father Torraco), immigration, etc are on the same moral level as abortion. They are not. As I said, the justification for voting for a pro-abortion candidate (but not in support of his pro-abortion policies) is a very high bar. In fact I'd be willing to believe that the only comparable moral issues are euthanasia and genocide, and, possibly, religious freedom, depending on the extremity of the position (for instance, a candidate who runs on a platform that he will require all citizens to make sacrifices to Ba'al, but who is pro-life (which, if we're being historically honest, is a ridiculous scenario) might justify voting for the pro-religious-liberty candidate who also supports abortion).