Voting In Catholic Teaching
... concerning areas or realities that involve fundamental ethical duties - legislative or political choices contrary to Christian principles and values are proposed or made, the Magisterium teaches that "a well-formed Christian conscience does not permit one to vote for a political programme or an individual law which contradicts the fundamental contents of faith and morals" (Compendium of the Social Doctrine of the Church n. 273)
By virtue of supernatural faith in God, in Christ and in the Lord's promises to the Church, Catholic are obliged to follow the guidance of the Magisterium in matters of faith and morals. With the advent of modern democracies, a theology of Christian voting has steadily built up, first through the work of theologians–forming a common Catholic moral tradition, then, through positive teaching of the Magisterium. Not all issues have been addressed by the Magisterium, and in such cases common teaching should be relied upon as representing the consensus of the theological tradition. In these answers on voting, this moral theology tradition will be relied upon when the Magisterium has not specifically taught.