When should Catholic bishops deny Communion?

Excerpts from:
http://chiesa.espresso.repubblica.it/articolo/206336?eng=y

... Neither Burke nor Chaput is new to taking stances like these. In 2004, on the eve of the previous presidential election, Burke advocated withholding communion from the Democratic candidate for the White House at the time, John Kerry, also a "pro-choice" Catholic.

In June of that year, from Rome, then-cardinal Joseph Ratzinger had sent to the United States bishops' conference a memorandum stating the "general principles" on this question.

Ratzinger's memorandum was private, but www.chiesa published it in its entirety. It sided with the unyielding bishops like Burke and Chaput. But most of the bishops in the United States were against withholding communion from Catholic politicians who support abortion. There were even two authoritative cardinals from the conservative wing, Francis E. George of Chicago and the Jesuit theologian Avery Dulles, who said they were reluctant to "make the Eucharist a political battleground." In the end, the bishops' conference decided to "apply" the principles presented by Ratzinger on a case-by-case basis, leaving it "to each bishop to express prudent pastoral judgments in his own specific circumstances."

From Rome, Cardinal Ratzinger accepted this solution and said that it was "in harmony" with the general principles of his memorandum.

In this, Ratzinger adopted a practice typical of Catholic countries in Europe, where rigorous principles coexist with more flexible pastoral customs....

Worthiness to Receive Holy Communion. General Principles
by Joseph Ratzinger, June 2004

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" [2000], 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.]