I’m often asked what a voter can morally do if two opposing candidates both support abortion. I recommend asking a simple question: Which of the two candidates will do less harm to unborn children if elected?
For example, is either of the candidates willing at least to ban partial-birth abortion? Is either of them willing to put up some roadblocks to free and easy abortion? Will either support parental notification, or parental consent, or waiting periods? Has either of them expressed a desire to ban late-term abortion, or to support pregnancy assistance centers? How about stricter regulation of abortion facilities? Has either candidate expressed support for that idea? Nobody is saying that's the final goal. But ask these questions just to see whether you can see any benefit of one of the candidates above the other.
One of the two of them will be elected; there is no question about that. So you are not free right now, in this race, to really choose the candidate you want. Forces beyond your control have already limited your choices. Whichever way the election goes, the one elected will not have the position we want elected officials to have on abortion.
In this case, it is morally acceptable to vote for the candidate who will do less harm. This is not "choosing the lesser of two evils." We may never choose evil. But in the case described above, you would not be choosing evil. Why? Because in choosing to limit an evil, you are choosing a good.
You can have a clear conscience in this instance, because you know that no law can legitimize even a single abortion, ever. If the candidate thinks some abortion is justified, you don't agree. Moreover, you are doing the most you can to advance the protection of life.
By your vote, you can keep the worse person out. And trying to do that is not only legitimate, but good. Some may think it's not the best strategy. But it is morally permissible.
Cardinal John O’Connor, in a special booklet on abortion, once wrote about this problem, “Suppose all candidates support ‘abortion rights’? … One could try to determine whether the position of one candidate is less supportive of abortion than that of another. Other things being equal, one might then morally vote for a less supportive position.” (1990, “Abortion: Questions and Answers”).
What if there’s a third candidate who does not have a strong base of support but does have the right position? Of course, we should work like crazy to build up that person’s base of support to make him or her electable. But that is not done on Election Day. That takes years of work, which should start now.
Meanwhile, remember that your vote is not a vote for canonization. It is a transfer of power. We can vote for a less than perfect candidate because we aren’t using our vote to make a statement, but to help bring about the most acceptable results under the circumstances.