Any country with a democratically-flavored polity gives its citizens the right to participate in the political process, typically through voting. Without getting into the moral or civic imperative of whether or not to vote (let's all exchange our voting rights for ipods--woohoo), I imagine most people would agree that voting decisions should be based on beliefs and opinions. Religion is (or, at least, should be) a major influence over one's beliefs and opinions. The religion of a political candidate should make a difference. The religion of the voter should make a difference. The frequently-proclaimed divorce between Church and State exists in the public sphere and not in the individual person. As an integral person, it's psychologically unhealthy to split the human identity into public and private halves.
One of the beauties of the Catholic faith has been the consistency with which it has provided foundational moral principles for making sound decisions. I really look forward to the USCCB publication of a document entitled "Forming Consciences for Faithful Citizenship: A Call to Political Responsibility From the Catholic Bishops of the United States". (I think it's great that there seems to be such a consensus on this document among U.S. bishops: the final vote was 221-4.) A shorter version of the 26-page document will be included in all parish bulletins. Stay tuned.
Recently the Bishop of Boston, Bishop O'Malley, explicitly stated that Catholics voting for pro-abortion politicians "border on scandal." Some bloggers and national newspapers find it contradictory that the Catholic Church states that voting for a pro-abortion politician can endanger the voter's eternal salvation. Needless to say, it's doctrinally false and unchristian to decry those in the wrong (anyone voting for a pro-abortion political candidate, anyone living in the state of mortal sin, etc.) of "going to hell". We should leave such judgment to God. I am sure that anyone believing that the Bishop said this misunderstood or twisted the meaning of his words. It's wonderful that the Catholic Church continues to clearly define the Catholic voter's duty and responsibility. There should be no room in the Catholic voter's mind when it comes to issues of life.
I also like the fact that Bishop O'Malley took umbrage with the Republican stance on various issues (immigration policy, capital punishment, economic issues, and the war in Iraq). Our primary duty as a voter is to our conscience, not to a political party. It's increasing difficult for me to identify with either political party. (I put myself in the independent political camp.) We should vote by issues, not by platforms. However, despite Bishop O'Malley's disagreement with the Republican party on the above-mentioned issues, he claims that abortion is a more important guide for Catholic voters.