Catechism of the Catholic Church §1901 — excerpt
If authority belongs to the order established by God, "the choice of the political regime and the appointment of rulers are left to the free decision of the citizens."
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: GS 74 # 3