McCain-Feingold wasn't just about expenditures. The government argued it could ban or confiscate books/movies based on who paid for them, or a tiny proportion of candidate/issue advocacy during election season. The first three paragraphs of this piece, by a major McCain-Feingold opponent, capture the overbroad reach of the FEC's claims:
You're right. I am not trying to argue that McCain-Feingold is constitutional. Rather, I'm trying to point out that a literal interpretation of the constitution in this case is invalid. It's a simplistic argument that sounds good at first but doesn't hold up under critical analysis. I just did some recent edits to better express my point of view.
http://www.nationalaffairs.com/publications/detail/the-myth-...