From The Vatican

The Church doesn't get any clearer. As Catholics, we have an obligation to defend the dignity of the human person. We also are obliged to inform ourselves and others in regards to the dignity of the human person. The Congregation for the Doctrine of Faith, with the approval of Pope John Paul II, specifically addressed our responsibilities as Catholics when it comes to politics. This is an excerpt of the Doctrinal Note on some questions regarding The Participation of Catholics in Political Life:

The Church recognizes that while democracy is the best expression of the direct participation of citizens in political choices, it succeeds only to the extent that it is based on a correct understanding of the human person.[17] Catholic involvement in political life cannot compromise on this principle, for otherwise the witness of the Christian faith in the world, as well as the unity and interior coherence of the faithful, would be non-existent. The democratic structures on which the modern state is based would be quite fragile were its foundation not the centrality of the human person. It is respect for the person that makes democratic participation possible. As the Second Vatican Council teaches, the protection of «the rights of the person is, indeed, a necessary condition for citizens, individually and collectively, to play an active part in public life and administration.

Catholics, in this difficult situation, have the right and the duty to recall society to a deeper understanding of human life and to the responsibility of everyone in this regard. John Paul II, continuing the constant teaching of the Church, has reiterated many times that those who are directly involved in lawmaking bodies have a «grave and clear obligation to oppose» any law that attacks human life. For them, as for every Catholic, it is impossible to promote such laws or to vote for them.

Read the entire text here: The Participation of Catholics in Political Life