[...] 5. 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. (Worthiness to Receive Holy Communion. General Principles, by Joseph Ratzinger) 2. 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 authorise 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).
Do you also need a source for omission/commission distinction?
I can extrapolate from that which you provided.