St. John Paul II was very clear in his encyclical Evangelium Vitae regarding which way Catholics should vote on pro-abortion laws.
When a measure appears on a ballot that is positively for the promotion or legalization of abortion, Catholics have a very clear example to follow in St. John Paul II.
St. John Paul II wrote extensively about life issues in his encyclical, Evangelium Vitae, and states clearly what the Church believes when it comes to voting for pro-abortion laws.
Abortion and euthanasia are thus crimes which no human law can claim to legitimize … In the case of an intrinsically unjust law, such as a law permitting abortion or euthanasia, it is therefore never licit to obey it, or to “take part in a propaganda campaign in favor of such a law, or vote for it.”
If a ballot measure is proposed that seeks to legalize abortion, a Catholic, according to St. John Paul II, must vote against it.
There are cases when laws are proposed that seek to limit abortion, and even if they don’t fully outlaw abortion, St. John Paul II believed a Catholic could vote for such laws.
[W]hen 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.
St. John Paul II firmly believed that abortion should have no place in civil society and that Catholics are called to be witnesses of hope in the world, supporting women and children in any way that they can.