If a Catholic votes for a pro-abortion candidate, when there is a pro-life choice available, does he automatically excommunicate himself from the Church?
In short, no, Catholics who votes for a pro-abortion candidate are not excommunicated. They are involved in actions that are gravely wrong but they have not been excommunicated.
Excommunication is a specific penalty for specific crimes in canon law.
The relevant canon for abortion and excommunication is canon 1398:
A person who procures a completed abortion incurs a latae sententiae excommunication.
In canon law, if a canon attaches a penalty to an action then that canon is to be interpreted strictly (canon 18). A voter who votes for a pro-abortion candidate has not procured an abortion and thus cannot suffer the penalty attached here.
Another relevant canon would be:
1329 §2. Accomplices who are not named in a law or precept incur a latae sententiae penalty attached to a delict if without their assistance the delict would not have been committed, and the penalty is of such a nature that it can affect them...
Once again, a penalty canon must be interpreted strictly. If John Doe did not vote for candidate X, would Jane Doe's specific abortion have taken place? It seems next to impossible to establish John Doe's vote as a necessary action for Jane Doe to have her specific abortion. Even if candidate X won by one vote and legalized abortion, John Doe's vote simply made abortion possible, he did not directly act as accomplice in Jane Doe's specific abortion and thus is not canonically considered a necessary collaborator.
To those wanting the immediate excommunication of pro-abortion politicians, I have to say that canon law simply does not read that way. To make a long story short, an excommunication for abortion has to be linked to a specific abortion and, given the structure of American government and medical institutions, one simply can't link a given legislator's vote with a specific abortion within the limits of causality recognized by canon law. Now, I like to think that the words of law generally mean what they say. If we distort the words of penal canon law to the degree necessary to make legislators fall within the present terms of the abortion canon, we would do violence to the text of the law, and that's always bad; distortions in law tend to come back and haunt us in other contexts.
Catholic politicians who vote pro-abortion, while committing a gravely sinful act, do not fall under these canons. So if the politician who votes pro-abortion is not excommunicated it is impossible to establish that the person who voted for them is excommunicated.
As previously mentioned, to vote to continue legalized abortion is grave matter. Someone who votes for such a thing may be guilty of a mortal sin (if they know and understand what they are doing). Such individuals need to repent and seek the grace of confession before receiving the Eucharist.