From Grace MacKinnon:
The crucial part in the whole matter is the proportionate reasons. When a Catholic voter casts his/her vote for a political candidate who advocates abortion and/or euthanasia, he or she must have a grave reason for doing so, and in this case, it must be equal (proportionate) in weight to the most fundamental life issue, which is abortion. Then, that issue would be a proportionate reason to vote for that candidate. Every year, abortion is the killer of 1.3 million innocent human beings. I personally cannot think of any other life issue that could be proportionate to that.
A possible example of morally permissible remote material cooperation would be if there were only two candidates running for the same office and both were in favor of abortion. Then, we would have the proportionate reason necessary to vote for one of them, as one of them will be elected anyway, and we would want the one who will support the least abortions to win. But this changes if one of the two is a candidate who is clearly opposed to abortion and/or euthanasia. Yes, a Catholic must vote according to his/her conscience, but the Church teaches that that conscience must be informed! A Catholic with a correctly informed conscience is morally bound always to defend life, and most especially innocent life.
To say that all the life issues in the election are equal would be wrong. They are not! In fact, in the very same memo we are speaking about, Cardinal Ratzinger states, "Not all moral issues have the same moral weight as abortion or euthanasia...there may be a legitimate diversity of opinion even among Catholics about waging war and applying the death penalty, but not however with regard to abortion and euthanasia." [emphasis added]
The US Bishops, basing themselves on the sound and consistent teaching of the Catholic Church, have stated, "Abortion and euthanasia have become pre-eminent threats to human life and dignity because they directly attack life itself, the most fundamental good and the condition for all others" (Living the Gospel of Life). In other words, human life is, yes, one among many life issues, but it is above all the rest and all other issues depend on it.