From Congregation for The Doctrine of Faith - “CONSIDERATIONS REGARDING PROPOSALS
TO GIVE LEGAL RECOGNITION
BETWEEN HOMOSEXUAL PERSONS”
"IV. POSITIONS OF CATHOLIC POLITICIANS
WITH REGARD TO LEGISLATION IN FAVOUR
OF HOMOSEXUAL UNIONS
If it is true that all Catholics are obliged to oppose the legal recognition of homosexual unions, Catholic politicians are obliged to do so in a particular way, in keeping with their responsibility as politicians. Faced with legislative proposals in favour of homosexual unions, Catholic politicians are to take account of the following ethical indications.
When legislation in favour of the recognition of homosexual unions is proposed for the first time in a legislative assembly, the Catholic law-maker has a moral duty to express his opposition clearly and publicly and to vote against it. To vote in favour of a law so harmful to the common good is gravely immoral.
When legislation in favour of the recognition of homosexual unions is already in force, the Catholic politician must oppose it in the ways that are possible for him and make his opposition known; it is his duty to witness to the truth. If it is not possible to repeal such a law completely, the Catholic politician, recalling the indications contained in the Encyclical Letter Evangelium vitae, “could licitly support proposals aimed at limiting the harm done by such a law and at lessening its negative consequences at the level of general opinion and public morality”, on condition that his “absolute personal opposition” to such laws was clear and well known and that the danger of scandal was avoided.(18) This does not mean that a more restrictive law in this area could be considered just or even acceptable; rather, it is a question of the legitimate and dutiful attempt to obtain at least the partial repeal of an unjust law when its total abrogation is not possible at the moment"
Bold text by me.