From another article:
Montana’s law had been upheld by the state Supreme Court after being challenged by a conservative group, American Tradition Partnership. But in a 5-4 ruling, the Supreme Court summarily reversed the lower court’s decision and invalidated the longstanding legislation.
The court determined in Citizens United that “independent expenditures, including those made by corporations, do not give rise to corruption or the appearance of corruption.” Montana’s attorney general claimed that the 1912 Corrupt Practices Act should stand as proof that the court’s argument had already been proven wrong over 100 years earlier, when the state’s mining billionaires blatantly bought off state legislators to further their interests. Most notoriously, copper king William Clark was appointed to the Senate by the state legislature, only to have his seating blocked by the Senate in Washington, D.C. over widespread evidence of bribery.
at the naiveté of the 5 conk judges.
“Even if I were to accept Citizens United, this Court’s legal conclusion should not bar the Montana Supreme Court’s finding, made on the record before it, that independent expenditures by corporations did in fact lead to corruption or the appearance of corruption in Montana,” Breyer wrote. “Given the history and political landscape in Montana, that court concluded that the State had a compelling interest in limiting independent expenditures by corporations.”
“Montana’s experience, like considerable experience elsewhere since the Court’s decision in Citizens United, casts grave doubt on the Court’s supposition that independent expenditures do not corrupt or appear to do so,” Breyer wrote.
Breyer gets it.
Money doesn't corrupt. It's speech!
By the way, if you want a dose of superficial constitutional theory, read Breyer's Active Liberty. It won't take long, and it reveals the lack of genuine intellectual support for the "living , breathing Constitution" types out there in Donkland. 
