Pope Benedict XVI (then Cardinal Ratzinger):

“The Church teaches that abortion or euthanasia is a grave sin. The Encyclical Letter Evangelium vitae, with reference to judicial decisions or civil laws that authorize or promote abortion or euthanasia, states that there is a ‘grave and clear obligation to oppose them by conscientious objection. [...] In the case of an intrinsically unjust law, such as a law permitting abortion or euthanasia, it is therefore never licit to obey it, or to ‘take part in a propaganda campaign in favour of such a law or vote for it’ (no. 73). Christians have a ‘grave obligation of conscience not to cooperate formally in practices which, even if permitted by civil legislation, are contrary to God’s law. Indeed, from the moral standpoint, it is never licit to cooperate formally in evil. [...] This cooperation can never be justified either by invoking respect for the freedom of others or by appealing to the fact that civil law permits it or requires it’ (no. 74).

Not all moral issues have the same moral weight as abortion and euthanasia. For example, if a Catholic were to be at odds with the Holy Father on the application of capital punishment or on the decision to wage war, he would not for that reason be considered unworthy to present himself to receive Holy Communion. While the Church exhorts civil authorities to seek peace, not war, and to exercise discretion and mercy in imposing punishment on criminals, it may still be permissible to take up arms to repel an aggressor or to have recourse to capital punishment. 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.

. . . .

Regarding the grave sin of abortion or euthanasia, when a person’s formal cooperation becomes manifest (understood, in the case of a Catholic politician, as his consistently campaigning and voting for permissive abortion and euthanasia laws), his Pastor should meet with him, instructing him about the Church’s teaching, informing him that he is not to present himself for Holy Communion until he brings to an end the objective situation of sin, and warning him that he will otherwise be denied the Eucharist.

When ‘these precautionary measures have not had their effect or in which they were not possible,’ and the person in question, with obstinate persistence, still presents himself to receive the Holy Eucharist, ‘the minister of Holy Communion must refuse to distribute it’ (cf. Pontifical Council for Legislative Texts Declaration ‘Holy Communion and Divorced, Civilly Remarried Catholics’ [2002], nos. 3-4). This decision, properly speaking, is not a sanction or a penalty. Nor is the minister of Holy Communion passing judgment on the person’s subjective guilt, but rather is reacting to the person’s public unworthiness to receive Holy Communion due to an objective situation of sin.

[N.B. 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 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.]