There are many reasons to oppose abortion and to vote against pro-abortion candidates: The barbarism of rupping apart unborn children; the dreadful impact on the future mental health of many of the women involved; the dire effect of a shortage of children on individuals and on society as a whole, and so on. But, most of all, it is a SIN!
According to Church teaching, voting for a child-killing amendment is formal cooperation in evil, a deliberate decision to deprive an innocent human being of his life, which, as Pope John Paul II taught in Evangelium Vitae, is “a grave act of disobedience to the moral law.”
The deliberate decision to deprive an innocent human being of his life is always morally evil and can never be licit either as an end in itself or as a means to a good end. It is in fact a grave act of disobedience to the moral law, and indeed to God himself, the author and guarantor of that law; it contradicts the fundamental virtues of justice and charity. – Pope St. John Paul II, Evangelium Vitae
Further on in this same encyclical, he adds:
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 favour of such a law, or vote for it.”
Pope Benedict XVI further clarified the moral implications and underscored the severe gravity of such an action when he wrote:
A Catholic would be guilty of formal cooperation in evil, and so unworthy to present himself for Holy Communion, if he were to deliberately vote for a candidate precisely because of the candidate’s permissive stand on abortion and/or euthanasia. – Pope Benedict XVI, Worthiness to Receive Holy Communion: General Principles
Here, Pope Benedict specifically speaks of voting for a candidate, but a clear-cut vote for a child-killing ballot measure, as is a vote for Amendment 4, would certainly also make a Catholic guilty of formal cooperation in evil, make him unworthy for Holy Communion, and jeopardize his eternal salvation without sacramental Confession.
In Ireland and in the USA, we are entering election season. You know what to do!