I have heard that it is a sin to vote for a pro-abortion candidate. Is that true? Would the sin be mortal?
It depends on the reason why a person votes for the candidate who supports abortion. In his letter, Worthiness to Receive Holy Communion, then-Cardinal Joseph Ratzinger (Pope Emeritus Benedict XVI), who was at the time head of the Congregation for the Doctrine of the Faith, wrote:
A Catholic would be guilty of formal cooperation in evil, and so unworthy to present himself for holy Communion, if he were to deliberately vote for a candidate precisely because of the candidate’s permissive stand on abortion and/or euthanasia. When a Catholic does not share a candidate’s stand in favor of abortion and/or euthanasia, but votes for that candidate for other reasons, it is considered remote material cooperation, which can be permitted in the presence of proportionate reasons.