I’ll take a run at that. Some have said that “proportionate reasons” mean that the other candidate is even worse on pro-choice. But I don’t think that interpretation makes sense. If Cardinal Ratzinger had meant that, he could have said so in much clearer, directly language. Cardinal Ratzinger is very intelligent and a good writer. I’m sure he chose his words to mean exactly what they say. The most straightforward interpretation, then, is that the voter takes into account other reasons, that he feels are proportionate to the reason for voting for the pro-life (or lesser pro-choice) candidate. At this point many say that without the right to life, no other right means anything, and that there can be no defensible reason for valuing some other good higher than preventing abortion. To this I say again that Cardinal Ratzinger had considered his words very carefully. He knew full well that the right to life is foundational. And if he thought there could be no circumstance under which a faithful Catholic could apply his exception clause, then he would not have risked confusion by putting in an exception clause that could never be used. All this is preliminary to establish that in Cardinal Ratzinger’s mind, it must be possible for proportional reasons to exist.
As to what they might be, here is just one example: Suppose that in a particular race, the pro-life candidate in question has a very low chance of being able to make any change in abortion law. All things being equal, we would still be required to vote for him. But suppose not all things are equal, and there is another issue at stake in which the holder of this office has a much better chance of making a difference. And suppose the candidate with the preferred position on that issue also happens to be pro-choice. Voting for him will likely achieve the lesser goal, while voting for the pro-life candidate would sacrifice the lesser goal and still may not achieve the greater goal of preventing abortion. Faced with this choice, it is conceivable that a voter may decide that more good can be done by voting for the candidate who happens to be pro-choice. Such a decision would not be taken lightly, but I can see (and I think so could Cardinal Ratzinger) that such a choice might be possible.