No. A Catholic belonging to a party that supports sin in a broad sense in its party platform is not excommunicated. A Catholic who belongs to a Socialist, Communist, Nihilist or Freemasonic group (or similar type groups) would be. The key distinction is that these groups by their very nature and intention are contrary to the Catholic church or to civic society. Sin broadly speaking does affect civic society and the Church because all mortal sin is contrary to justice and charity and thus contrary to the peace and concord of society, but canon law doesn't penalize all moral faults but only the most severe harms to Truth and the Church; Note for example, that abortion is a canonical crime punished by excommunication but murder, rape or robbery are not generally punished so.
Can 1374 in the 1983 Code (quoted in the question) is specifically about associations which plot against the Church or civic powers. Canon 2335 of the 1917 Code includes the Masons explicitly but otherwise has the same qualifications.
We cannot claim these canons mean any group which "supports sin" falls under the same penalty due to the requirement in Canon Law that canons pertaining to punishments must be interpreted strictly.
Can. 18 Laws which establish a penalty, restrict the free exercise of rights, or contain an exception from the law are subject to strict interpretation.
Correspondingly, canons which describe rights or privileges are interpreted broadly for the same reasons.
Furthermore, the claim of "supports sin" is vague and problematic. As asked by Mike Borden in the comments "Is there a political party which does not support sins in its party platform?", we would be inclined to answer "no" because in so far as we are fallen, we fall short of the ideal of morality. The fault may be slight. If the canon applied to "supporting sin" generically, that would include venial, which would be excessive penalty and result in a huge swathe of people being excommunicated.
Canonists do not conclude that politicians formally cooperate with abortion by legalizing it and thus would not automatically be excommunicated (which is not the same as saying their votes for abortion are not sinful):
"formal cooperation" involves the doctor who performs the abortion, the nurse who assists, the woman having the abortion, the man who agrees to take his wife or girlfriend, the person who drives the woman to the clinic--and even that last one is questionable. With legislators, what you have is probably best termed "material cooperation." - Phil Gray (quoted in "Just Punishment", by Brian O'Neel)
Canonists are also clear that not all support for abortion falls under the excommunication of Canon 1329, hence, if not all support for that sin falls under the excommunication, neither can support for any sin as a political platform fall under the penalty of Canon 1374.
People who make that argument [voting for taxpayer funding of abortion is a specific act] are possibly using Canon 1329 [which prescribes excommunication for accomplices in such offenses as abortion](square brackets in original), and are trying to characterize these people as accomplices. The problem is that in order to be an accomplice to an action, you have to be able to show that, but for this vote that freed up X number of dollars, this abortion would not have taken place. What if the bill passes by two votes? Whose vote do you blame? - Ed Peters (quoted in "Just Punishment", by Brian O'Neel)