Mark A.R. Kleiman asks an interesting question about when a lay Catholic can vote for a politician who disagrees with an article of official doctrine and when they can't.

I'm not an expert, but I follow this issue, partly because I wonder if it could lead to a backlash. Many Protestants who are gleeful now would get very upset if this was extended to not voting for Protestants.

There is some ambiguity as to what is a sin and what isn't. There are old rules about when a fetus quickens left over from before the Church signed on to 'no abortion at any time'. As far as I know, these ideas of St. Augustine have never been officially repudiated, but merely ignored.

The first papal canon, Effraenatam, that universally imposed a penal penalty of excommunication for abortion was issued by Sixtus V in 1588. It applied to all abortions and was reserved to the Holy See. In 1591, the law was modified by Gregory XIV so that the penalty would not apply when a fetus was not "animated" or "ensouled" under the Aristo-Aquinan theory of when human life begins (not before 40 days) and gave the local bishops control of these cases. This was motivated, at least in part, by the sheer volume of litigation the law had produced ("reserved to the Holy See" meant that each case had to be taken to Rome for the excommunication to be lifted). In 1869, Pius IX rescinded the animation exception. The canons of the 1917 and 1983 Codes apply to all direct abortions. Abortions incident to otherwise lawful medical care that is required to save the life of the mother (e.g. chemotherapy, hysterectomy of a cancerous uterus) are given an interpretive exception from the rule under the priciple of "double effect."

Given that something is a sin, when is a Catholic allowed to vote for a politician who supports it? If the politician supports other stands the Church favors? If they are not 'allowed' to vote for someone because of their opinion on one issue, when is it a mortal sin to disobey, and when is it a venial sin?

I've never seen official answers to these questions, and if there were any it would have been harder for Kennedy to say that as President he would not speak for the Church - and they would not speak for him. But since the question is not addressed in any Papal bulls, there is no law against an Archbishop declaring that a certain vote is a mortal sin.

Archbishop Raymond L. Burke will issue a pastoral letter addressing more fully questions raised by his reported statements last week that Catholics commit a mortal sin by knowingly voting for a candidate who advocates abortion.

On the other hand, Tony Blair is pro-choice but:

Even so, the Vatican itself may have played host to some unplanned intercommunion. Last February 22, British Prime Minister Tony Blair may—or may not—have been given Communion at a private papal Mass he attended with his Catholic wife and children. Neither the Vatican nor Downing Street is saying what actually happened, and people who were present give contradictory accounts. In Ecclesia de Eucharistia John Paul notes that the practice is permitted in special circumstances “to meet a grave spiritual need for the eternal salvation of an individual believer.” But corporate intercommunion “remains impossible,” he says, pending the reunion of separated churches.

As opposed to abortion as the Pope is, he doesn't seem convinved Blair is headed for hell unless he changes his mind, let alone those who vote for him.