Cardinal Ratzinger, who became Pope Benedict XVI in 2005 and served through 2013, wrote a blistering letter in 2004 forbidding Catholics from voting for pro-abortion Hillary Clinton or anyone who espouses her ideas.
The letter reads:
The Church teaches that abortion or euthanasia is a grave sin. The Encyclical Letter Evangelium vitae, with reference to judicial decisions or civil laws that authorize or promote abortion or euthanasia, states that there is a “grave and clear obligation to oppose them by conscientious objection. […] In the case of an intrinsically unjust law, such as a law permitting abortion or euthanasia, it is therefore never licit to obey it, or to ‘take part in a propaganda campaign in favour of such a law or vote for it’” (no. 73). Christians have a “grave obligation of conscience not to cooperate formally in practices which, even if permitted by civil legislation, are contrary to God’s law. Indeed, from the moral standpoint, it is never licit to cooperate formally in evil. […] This cooperation can never be justified either by invoking respect for the freedom of others or by appealing to the fact that civil law permits it or requires it” (no. 74).
Cardinal Ratzinger reinforces his opinion in the footnote:
[N.B. 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.] Fr. Jay Scott Newman reinforced Cardinal Ratzinger’s letter in 2008:
Voting for a pro-abortion politician when a plausible pro-life alternative exists constitutes material cooperation with intrinsic evil, and those Catholics who do so place themselves outside of the full communion of Christ’s Church and under the judgment of divine law.
And Father John Lankeit’s homily leaves no doubt.
All six paragraphs of the letter plus half of the footnote make it clear that Catholics who support abortion by voting for pro-abortion candidates are cooperating in evil and may not receive Communion.