Read the reference in Antoninus's #28.
Pope John Paul’s 1995 encyclical on bioethics, Evangelium Vitae. When outlawing abortion is politically impossible, the pope held, a politician could vote for a law that permits some abortions, if it’s the most restrictive result feasible and the alternative would be a more liberal standard.
When it is not possible to overturn or completely abrogate a pro-abortion law, an elected official, whose absolute personal opposition to procured abortion was well known, 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,” the pope wrote. “This does not in fact represent an illicit cooperation with an unjust law, but rather a legitimate and proper attempt to limit its evil aspects.”