Except in the case in which a voter is faced with all pro-abortion candidates, * a candidate that is pro-abortion disqualifies himself from receiving a Catholic’s vote. This is because being pro-abortion cannot simply be placed alongside the candidate’s other positions on Medicare and unemployment, for example; and this is because abortion is intrinsically evil and cannot be morally justified for any reason or set of circumstances. To vote for such a candidate even with the knowledge that the candidate is pro-abortion is to become an accomplice in the moral evil of abortion. If the voter also knows this, then the voter sins mortally.
* A voter may likewise vote for that candidate who will most likely limit the evils of abortion or any other moral evil at issue. https://www.ewtn.com/vote/brief-catechism.asp