During the 2004 presidential election, which featured a Roman Catholic candidate, Sen. John Kerry, then-Cardinal Joseph Ratzinger, Prefect of the Congregation for the Doctrine of the Faith, wrote a letter to Cardinal Theodore McCarrick, who was then the archbishop of Washington, DC and the chair of an ad hoc committee of the USCCB that was, at the request of their fellow bishops, examining the issue of Catholic polticians whose public record on abortion, despite their personal and moral opposition, was at odds with church teaching, the text follows. It give us principles for using proportionate reasoning to arrive at prudential judgments. Hence, we may never formally cooperate with evil, but remote material cooperation is allowed for proportionate reasons.
Worthiness to Receive Holy Communion. General Principles
1. Presenting oneself to receive Holy Communion should be a conscious decision, based on a reasoned judgement regarding one's worthiness to do so, according to the Church's objective criteria, asking such questions as: "Am I in full communion with the Catholic Church? Am I guilty of grave sin? Have I incurred a penalty (e.g. excommunication, interdict) that forbids me to receive Holy Communion? Have I prepared myself by fasting for at least an hour?" The practice of indiscriminately presenting oneself to receive Holy Communion, merely as a consequence of being present at Mass, is an abuse that must be corrected (cf. Instruction "Redemptionis Sacramentum," nos. 81, 83).
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 propoganda 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).
3. 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.
4. Apart from an individuals's judgement about his worthiness to present himself to receive the Holy Eucharist, the minister of Holy Communion may find himself in the situation where he must refuse to distribute Holy Communion to someone, such as in cases of a declared excommunication, a declared interdict, or an obstinate persistence in manifest grave sin (cf. can. 915).
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.
6. 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" , nos. 3-4). This decision, properly speaking, is not a sanction or a penalty. Nor is the minister of Holy Communion passing judgement 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.]
After receiving this requested clarification from the Holy See, the ad hoc committee, chaired by Cardinal McCarrick, issued a report to their fellow bishops. After receiving this report, the USCCB issued a Statement on Responsibilities of Catholics in Public Life, which Archbishop Niederauer quotes in his public response. To wit:
"The question has been raised as to whether the denial of Holy Communion to some Catholics in political life is necessary because of their public support for abortion on demand. Given the wide range of circumstances involved in arriving at a prudential judgment on a matter of this seriousness, we recognize that such decisions rest with the individual bishop in accord with the established canonical and pastoral principles. Bishops can legitimately make different judgments on the most prudent course of pastoral action. Nevertheless, we all share an unequivocal commitment to protect human life and dignity and to preach the Gospel in difficult times."Then, in 2006, reflecting further on the broader implications of these questions, the USCCB promulgated "Happy Are Those Who Are Called to His Supper": On Preparing to Receive Christ Worthily on the Eucharist.
By the way, let's not forget to fast one hour before Mass. To be safe, make it an hour before Mass begins. Otherwise, you're just guessing as to what time you may actually receive communion.