According to Roman Catholicism, is it a sin to vote for pro-choice politicians? Let's not make this a debate about pro-life and pro-choice views of abortion. According to Roman Catholic teaching, would it be a sin if I voted for a pro-choice politician such as Bernie Sanders?
Yes, if the candidate was voted for because they were pro-abortion

Pope Benedict XVI (formerly Cardinal Joseph Ratzinger) succinctly summed up the Catholic Church's teachings in the memorandum "Worthiness to Receive Holy Communion: General Principles":


  A Catholic would be guilty of formal cooperation in evil, and so unworthy to present himself for Holy Communion [due to being in a state of mortal sin], 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.


In other words:


Voting for a pro-abortion candidate because they are pro-abortion is a mortal sin
Voting for a pro-abortion candidate despite you not supporting that stance is remote material cooperation*, but may be permitted (i.e. not sinful or a venial sin) if you have proportionally strong reasons for doing so


* "Remote mediate material cooperation is cooperation by supplying the occasion or material for the commission of a morally wrong action, but is further removed from the immoral act itself. In the nursing home example above, the person who sells drugs to the nursing home would know the kind and quantity of drugs sold. She has no intention of providing the means for the nursing home to endanger patients by drugging them into a stupor to reduce staff requirements. She sells drugs to legitimate customers and provides any informational assistance necessary about the drugs. Her intention may be to earn a living in a field that can heal or reduce the suffering of others. Remoteness can vary by degrees, with one cooperative action being more remote than another." (Source)

So are Catholics obligated to vote for a pro-life candidate, regardless of their other positions?

Based on the above, a Catholic is not obligated to vote for a pro-life candidate if they have good reason not to, although that good reason should be weighed in proportion to abortion. So being pro-life does not automatically give the politician a free pass over any other moral failings or guarantee a Catholic vote.

To use an extreme example, say that a politician were against all forms of abortion, but planned to commit genocide against every minority ethnic group in their country. In that case, a Catholic would not be obligated to vote for them simply because they are pro-life, and would be permitted to vote for a pro-abortion candidate who does not advocate for such atrocities.  On the other end of the spectrum, if getting a tax break is your sole reason for voting for a pro-abortion candidate, it's unlikely to be considered a proportionate reason when compared to the killing of an unborn child, and the Church may consider that sinful.

What about voting or campaigning for a pro-abortion ballot initiative?

Voting specifically for a pro-abortion law on a ballot would be sinful, as would campaigning for such a position. From the same memorandum, which cites the encyclical Evangelium vitae:


  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' [...] 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