In the society which which we live, we have a Christian duty to act within our power to protect the most fundamental basis of our community---the right to life. We cannot ignore the image of God in every human being, including in those society deems unworthy of life. "What you do to the least of Me, you do to Me." We are not islands---we bear some responsibility for those around us.
So then, why doesn't the RCC put an equal burden on voting for a candidate who does not oppose the death penalty? Why do people not have a defective conscience if they vote for a politician who is in favor of it? That hasn't been made a matter of faith. And what about the millions of Iraqi civilians killed, not to mention the soldiers? But it seems protecting their lives isn't a matter of faith, either, and I can vote for a pro-war candidate with a clear conscience. No, the RCC in the US is playing politics, not protecting life.
Yes, we do have a Christian duty to live a Godly life. We do not have a Christian duty to impose Christian morality on other people. We are to be the light, the shining example. That itself presupposes that others will not be living the same way and will need the example. And, in fact, it is only the morality of the RCC and the Fundamentalists, not all Christians that they are trying to impose. The mainstream Protestant churches and the Orthodox Church quite rightly admits the existence of a grey area regarding abortion, when one has to balance the life of the unborn child against the life of the mother. In the RCC there is no balance, the life of the child trumps the life of the mother, and abortion should be illegal at any time and for any reason. It's back to the same thing again, rigid rules blindly enforced instead of wisdom administered with compassion.