In a 1912 ballot initiative, the citizens of Montana passed the Montana Corrupt Practices Act in response to the major influence of corporations on state elections.[5] During the early 20th Century, a number of rich "Copper Barons" controlled most of the political process through quid pro quo financial transactions with politicians.[6] In response, the state restricted the amount of money that corporations and individuals could donate to campaigns.[7] American Tradition Partnership (ATP), formerly known as Western Tradition Partnership,[8] had challenged the Montana Corrupt Practices Act of 1912...
The U.S. Supreme Court reversed the Montana Supreme Court without hearing oral argument in American Tradition Partnership, Inc. v. Bullock, 567 U.S. 516 (2012) by a short, per curiam opinion. The Court wrote only that "[t]here can be no serious doubt" that the holding of Citizens United applies to the Montana state law, as per U.S. Const., Art. VI, cl. 2, and that "Montana's arguments in support of the judgment below either were already rejected in Citizens United, or fail to meaningfully distinguish that case.
https://en.wikipedia.org/wiki/Western_Tradition_Partnership,_Inc._v._Montana