In 2004 when Pope Benedict was a Cardinal and head of the CDF he wrote a letter to the US Bishops on this topic. He wrote (emphasis added):
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 favour 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.
Since abortion is intrinsically evil there are very few ways to arrive at proportionate reasons. However, the example given by Dr. Delgado is indeed one of them. In the example he’s discussing the choice is between two evils (both candidates are pro-abortion), however one of them may be graver than the other (one may support more abortion “rights” or other evils that the other candidate does not support). In such a situation it would not be immoral to vote for one of the two pro-abortion candidates in order to lessen the evil that may result from the election.
In a Catholic News Service article this paragraph was similarly explained:
Vatican officials later said that defining what constitutes “proportionate reasons” for a Catholic in such cases might be extremely difficult. One possible example, they said, was when Catholic voters face a choice between two candidates who support legalized abortion but to widely differing degrees, and do not want to renounce their responsibility to vote.