You need to assess whether the reason or reasons you would vote for a pro-choice candidate are proportionate enough given the candidate’s pro-choice position.
Jimmy Akin highlights this:
Archbishop John Myers (Newark, NJ) has an article in the Wall Street Journal on what Ratzinger said regarding proportionate reasons for voting for a pro-abort candidate. Excerpts:
What are “proportionate reasons”? To consider that question, we must first repeat the teaching of the church: The direct killing of innocent human beings at any stage of development, including the embryonic and fetal, is homicidal, gravely sinful and always profoundly wrong. Then we must consider the scope of the evil of abortion today in our country. America suffers 1.3 million abortions each year–a tragedy of epic proportions.
Thus for a Catholic citizen to vote for a candidate who supports abortion and embryo-destructive research, one of the following circumstances would have to obtain: either (a) both candidates would have to be in favor of embryo killing on roughly an equal scale or (b) the candidate with the superior position on abortion and embryo-destructive research would have to be a supporter of objective evils of a gravity and magnitude beyond that of 1.3 million yearly abortions plus the killing that would take place if public funds were made available for embryo-destructive research.
Frankly, it is hard to imagine circumstance (b) in a society such as ours.
Certainly policies on welfare, national security, the war in Iraq, Social Security or taxes, taken singly or in any combination, do not provide a proportionate reason to vote for a pro-abortion candidate.
In June, Archbishop Burke had said Catholics who vote for pro-abortion politicians were committing a grave sin and must confess before receiving Communion.
He had told the Post-Dispatch that “it doesn’t make a lot of difference” why a Catholic votes for a pro-abortion politician. “If the voter is aware of that politician’s pro-abortion position, they would still be supporting someone, who is cooperating in the promotion of abortion,” he said.
But in an interview Thursday, Archbishop Burke told the Post-Dispatch that while he has not changed his position, he said he felt he had to clarify.
The archbishop told a reporter that he believes Catholics could vote for a politician who supports abortion rights as long as that’s not the reason they are voting for the candidate, and they believe the politician’s stance on other moral issues outweighs the abortion issue.
“That is called remote material cooperation and if the reasons are really proportionate, and the person remains clear about his or her opposition to abortion, that can be done,” the archbishop told the Post-Dispatch.
“The sticking point is this - and this is the hard part,” Archbishop Burke was quoted as saying. "What is a proportionate reason to justify favoring the taking of an innocent, defenseless human life? And I just leave that to you as a question. That’s the question that has to be answered in your conscience. What is the proportionate reason?
One of the reasons the bishop did not discuss this point in June is because “it is difficult to imagine what that proportionate reason would be,” he said.
Fr Stephen F. Torraco:
14. Is it a mortal sin to vote for a pro-abortion candidate?
Except in the case in which a voter is faced with all pro-abortion candidates (in which case, as explained in question 8 above, he or she strives to determine which of them would cause the let damage in this regard), a candidate that is pro-abortion disqualifies himself from receiving a Catholic’s vote. This is because being pro-abortion cannot simply be placed alongside the candidate’s other positions on Medicare and unemployment, for example; and this is because abortion is intrinsically evil and cannot be morally justified for any reason or set of circumstances. To vote for such a candidate even with the knowledge that the candidate is pro-abortion is to become an accomplice in the moral evil of abortion. If the voter also knows this, then the voter sins mortally.
Bishop Robert J. Carlson:
If one had a properly formed conscience admitting the grave evil of abortion and euthanasia, as the Church teaches, and does not share a candidates stand in favor of abortion and euthanasia, but votes for the candidate for other reasons, it is considered remote material cooperation which can be permitted, Cardinal Ratzinger states, if proportionate reasons are present, e.g., the candidate would limit abortions.