Since there are pro-lifers out there taking John Cardinal O'Connor out of context when it comes to how a serious pro-lifer could or should vote, I thought it might be helpful to provide his entire comment.
In his 1990 message, "Abortion Questions and Answers," we find this response:
Suppose all candidates support "abortion rights"?
In good conscience one could refrain from voting altogether. In some instances this might be best, even though voting is normally a moral obligation. Or 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.
If all candidates support 'abortion rights' equally one might vote for the candidate who seems best in regard to other issues, hoping that one day he or she could be persuaded to become pro-life.