The American idea is that the public interest does not arise from the ether and alight upon the shoulder of some wise Senator pulling his chin. It is defined by the rough clash of voices and forces and, yes, interests. Contrary to McCain, these clashing interests are good. The American idea has never been to suppress them but to pit them against each other and allow them to proliferate. The more they proliferate, the more they check and balance each other.
Coal fights natural gas.
Napster fights the record industry. Nader fights everybody. No one interest becomes paramount. McCain-Feingold would shackle private interests by severely restricting their ability to express themselves politically. There are few more important or more cherished ways for those outside the political system to express themselves than by contributing to a political party that reflects their views.