Are politicians who are integral to passing pro-abortion laws automatically excommunicated from the Catholic Church?
The short answer is no, but it does not end there.
Catholics who procure a completed abortion are subject to a latae sententiae excommunication. That means that the excommunication is not imposed by an authority or trial (as with a ferendae sententiae penalty); rather, being expressly established by canon law, it is incurred ipso facto when the delict is committed (a latae sententiae penalty). Canon law states that in certain circumstances "the accused is not bound by a latae sententiae penalty"; among the ten circumstances listed are commission of a delict by someone not yet sixteen years old, or by someone who without negligence does not know of the existence of the penalty, or by someone "who was coerced by grave fear, even if only relatively grave, or due to necessity or grave inconvenience."
According to a 2004 memorandum by Joseph Cardinal Ratzinger, Catholic politicians who consistently campaign and vote for permissive abortion laws should be informed by their priest of the Church's teaching and warned to refrain from receiving the Eucharist or risk being denied it until they end such activity. This position is based on Canon 915 of the 1983 Code of Canon Law and has also been supported, in a personal capacity, by Archbishop Raymond Leo Cardinal Burke, Prefect of the Apostolic Signatura, the highest judicial authority in the Catholic Church after the pope himself. Pope Francis reaffirmed this position in March 2013, when he stated that "[people] cannot receive Holy Communion and at the same time act with deeds or words against the commandments, particularly when abortion, euthanasia, and other grave crimes against life and family are encouraged. This responsibility weighs particularly over legislators, heads of governments, and health professionals.
During the Year of Mercy (2015) Pope Francis granted priests the grace of absolving the sin of abortion, without going to the local bishop to receive permission to first lift the automatic excommunication incurred by the abortion in the first place.
- Given this need, lest any obstacle arise between the request for reconciliation and God’s forgiveness, I henceforth grant to all priests, in virtue of their ministry, the faculty to absolve those who have committed the sin of procured abortion. The provision I had made in this regard, limited to the duration of the Extraordinary Holy Year, is hereby extended, notwithstanding anything to the contrary. I wish to restate as firmly as I can that abortion is a grave sin, since it puts an end to an innocent life. In the same way, however, I can and must state that there is no sin that God’s mercy cannot reach and wipe away when it finds a repentant heart seeking to be reconciled with the Father. May every priest, therefore, be a guide, support and comfort to penitents on this journey of special reconciliation. - Apostolic Le Misericordia et misera of the Holy Father Francis.
As Peter Turner mentioned in his answer the sin of such politicians who vote or publicly endorse abortions are guilty of scandal, which is not an excommunicable offence.
A politician who votes for evil laws, like expanding people's rights to kill babies, may in some people's eyes be accomplices to abortion, but the particular sin they're culpable for before the congregation is scandal.
Therefore, they are guilty of scandal who establish laws or social structures leading to the decline of morals and the corruption of religious practice, or to "social conditions that, intentionally or not, make Christian conduct and obedience to the Commandments difficult and practically impossible." - CCC 2286
Cardinal Ratzinger, as Prefect of the Congregation for the Doctrine Faith say politicians who publicly support abortions should be refused communion. That implies that such politicians are a serious source of scandal, but are not officially excommunicated. It is a fine line, but there it is.
Cardinal Ratzinger—in his official capacity as the prefect of the Congregation for the Doctrine of the Faith—sent a memorandum (July 2004) to Cardinal McCarrick, archbishop of Washington, regarding the worthy reception of the Eucharist. He writes:
Regarding the grave sin of abortion or euthanasia, when a person’s formal cooperation becomes manifest (understood, in the case of a Catholic politician, as his consistently campaigning and voting for permissive abortion and euthanasia laws), his Pastor should meet with him, instructing him about the Church’s teaching, informing him that he is not to present himself for Holy Communion until he brings to an end the objective situation of sin, and warning him that he will otherwise be denied the Eucharist.
Citing a declaration from the Pontifical Council for Legislative Texts, Cardinal Ratzinger continues:
When “these precautionary measures have not had their effect or in which they were not possible,” and the person in question, with obstinate persistence, still presents himself to receive the Holy Eucharist, “the minister of Holy Communion must refuse to distribute it….” This decision, properly speaking, is not a sanction or a penalty. Nor is the minister of Holy Communion passing judgment on the person’s subjective guilt, but rather is reacting to the person’s public unworthiness to receive Holy Communion due to an objective situation of sin.
In light of these teachings, it is likely that Archbishop Cupich either misspoke or was not sufficiently aware of Cardinal Ratzinger and Bergoglio’s teaching.
First, it is clear Catholic politicians who support abortion “cannot receive holy communion” due to their “objective situation of sin.” Moreover, the minister of Holy Communion “must refuse to distribute it” to them (emphasis added).
Second, while Archbishop Cupich is correct in saying the communion rail is not the place to discuss a person’s worthiness to receive Holy Communion, this is not what the Church in fact proposes. Rather, the Church instructs pastors to meet privately with the politician, instruct him on the Church’s teaching, and warn him that he will be denied the Eucharist unless “he brings to an end the objective situation of sin.” Thus, while the communion rail is not the place to have conversations, it is the place to refuse communion to Catholic politicians who support abortion. - On Giving Communion to Pro-Abortion Politicians
Canon 1398 is a norm of canon law of the Catholic Church which declares that "a person who procures a completed abortion incurs a latae sententiae (automatic) excommunication.
Canon 915, applies to Catholic legislators who make abortion legal, does not impose excommunication, but instead imposes on the minister of Holy Communion the obligation to refuse the sacrament to those who "obstinately persevere in manifest grave sin".
The July 2004 document of Cardinal Joseph Ratzinger has an interesting ending note that is truly an eye opener:
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. - Worthiness to Receive Holy Communion — General Principles