From the Vatican:

Doctrinal Note on some questions regarding the participation of Catholics in political life: "In this context, it must be noted also that a well-formed Christian conscience does not permit one to vote for a political program or an individual law which contradicts the fundamental contents of faith and morals. The Christian faith is an integral unity, and thus it is incoherent to isolate some particular element to the detriment of the whole of Catholic doctrine."