Fellow CKW editor, Christopher, writes an excellent post called Cardinal Ratzinger, Bishop Burke and "Proportionate Reasons."
My own thoughts are less precise than his. The problem many people experience with proportionate reasons is that they are in fact relativists. Nothing is proportional when everything has the same weight. But of course even relativists assign weights to favored causes. It is like liberating Bergen-Belsen and your first reaction is to call for the raising of the minimum wage for Jewish people. This is not to say that one issue only must be worked on at a time, instead it is to work to consistently bring us closer to the culture of life where the dignity of each person is respected. That respect is destroyed directly in the abortuaries, but Catholic social teaching doesn’t end after one’s escape from the surgical implement of an abortionist. Tom of Disputations also makes a good point on single issue voting.
My point is narrow: It’s wrong to criticize Catholic statements on politics for mentioning issues other than the right to life. Particular statements may be criticized for muddling the issues, but not simply for mentioning them. To do so is to fail both politically and morally(!); politically because you’ll never get what you want if you don’t tell anyone what you want; and morally because you have an obligation under justice to contribute to the common good by promoting Church teaching on the issues to the body politic.
Update: Jimmy Akin does an in depth post on cooperation and proportionate reasons. A must read with good examples for clarifications of the terms used.a