First of all, there is no absolute moral requirement to vote for a candidate for every office on a ballot. If you cannot in good conscience vote for any of the candidates, you are free to abstain from voting on that section of the ballot. You should still vote for any other office or measure for which there is an acceptable option. So, that means the priest’s choice to abstain from voting for an office in which the only candidates listed are pro-abortion is a valid option.
Secondly, in a race in which every candidate running holds views contrary to sound morals, it is possible to vote for the candidate whose views are closest to right morality. If one of the candidates is better on the life issues than the others – for example, one candidate favors at least some restrictions on abortion access, or opposes embryonic stem-cell research and human cloning – you could vote for that person over the candidates who are more at odds with right morals. And, also, if both major candidates are bad but one can be considered a graver threat than the other, you could choose to vote for one bad candidate to keep the candidate you consider a graver threat out of power.
What you cannot do is to vote for a candidate because he supports abortion, embryonic stem-cell research, human cloning, euthanasia, or any other measure that deliberately murders innocent human persons. Only if your vote is given as a means to promote anti-life, pro-death measures would it be an occasion of sin and something that must be confessed. I do think though that pro-life advocates need to be more careful in stating their voting strategies, so as not to make it seem that one particular pro-life voting strategy excludes the possibility of other valid pro-life voting strategies. As you discovered, lack of precision on this (especially from a priest) can lead fellow pro-lifers to suffer from false scruples.
