There are those who feel that it is acceptable to vote for a pro-abortion politician if their other positions appear to benefit the common good and help reduce the number of abortions. The problem with this position is that reducing the number of abortions, as important as this is, is not enough. If abortion could be completely eliminated in fact but was still legal, then the task is not finished. This is because the legality of abortion is itself an attack on the very foundations of the rule of law and the common good.
As Blessed Pope John XXIII taught in Mater et Magistra, "individual human beings are the foundation, the cause and the end of every social institution" (219). The legality of abortion is a direct assault on the foundation and purpose of the political community itself. The rule of law and the common good are meaningless concepts if innocent human beings are allowed to be legally murdered.
This teaching is reaffirmed and made even more explicit in Servant of God Pope John Paul II’s encyclical Evangelium Vitae: "Disregard for the right to life, precisely because it leads to the killing of the person whom society exists to serve, is what most directly conflicts with the possibility of achieving the common good" (72). One cannot truly work towards the common good by ignoring the legality of abortion. The legality of abortion necessarily undermines the common good.
A candidate who runs on a pro-abortion platform essentially states that he will not work towards the purpose of government, i.e. the common good. He renders his administration incapable of benefiting the common good in any way except accidentally. This is because the very nature his administration's position attacks the common good by enabling the legal attack on the lives of the unborn.
Any attempt to put forward a pro-abortion politician as an acceptable candidate fails. A candidate who promises to attack the very foundation and purpose of government, the good of life on which all the rest of the common good depends, can never be acceptable.