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 favors a policy promoting an intrinsically evil act, such as abortion, euthanasia, assisted suicide, deliberately subjecting workers or  the poor to subhuman living conditions, redefining marriage in ways that violate its essential meaning, or racist behavior, 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.

Forming Consciences for Faithful Citizenship, para. 34.