This excerpt is particularly apt (emphasis added):
Even if we understand the moral dimensions of the full array of social issues and have correctly prioritized those involving intrinsic evils, we still must make prudential judgments in the selection of candidates. In an ideal situation, we may have a choice between two candidates who both oppose public policies that involve intrinsic evils. In such a case, we need to study their approach on all the other issues that involve the promotion of the dignity of the human person and prayerfully choose the best individual.
Limiting Grave Evil
In another circumstance, we may be confronted with a voting choice between two candidates who support abortion, though one may favor some limitations on it, or he or she may oppose public funding for abortion. In such cases, the appropriate judgment would be to select the candidate whose policies regarding this grave evil will do less harm. We have a responsibility to limit evil if it is not possible at the moment to eradicate it completely.
The same principle would be compelling to a conscientious voter who was confronted with two candidates who both supported same-sex unions, but one opposed abortion and destructive embryonic research while the other was permissive in these regards. The voter, who himself or herself opposed these policies, would have insufficient moral justification voting for the more permissive candidate. However, he or she might justify resorting to a write-in vote or abstaining from voting at all in this case, because of a conscientious objection.
In 2004 a group of United States Bishops, acting on behalf of the USCCB and requesting counsel about the responsibilities of Catholic politicians and voters, received a memo from the office of Cardinal Joseph Ratzinger, the future Pope Benedict XVI, which stated: "A Catholic would be guilty of formal cooperation in evil, and so unworthy to present himself for Holy Communion, 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 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."
Could a Catholic in good conscience vote for a candidate who supports legalized abortion when there is a choice of another candidate who does not support abortion or any other intrinsically evil policy? Could a voter's preference for the candidate's positions on the pursuit of peace, economic policies benefiting the poor, support for universal health care, a more just immigration policy, etc. overcome a candidate's support for legalized abortion? In such a case, the Catholic voter must ask and answer the question: What could possibly be a proportionate reason for the more than 45 million children killed by abortion in the past 35 years? Personally, we cannot conceive of such a proportionate reason.