There's lots of good stuff out there about the Citizens United case, but this post by Michael Waldman struck me the most:
Why will this matter? Isn’t there a lot of money sloshing around in politics already? Consider Exxon-Mobil. In 2008, its political action committee (PAC) raised about $1 million from its employees and offices. Its profits that year -– which it was legally barred from pouring into politics -– were $45 billion. It was illegal for Exxon to spend that money on elections; now with this decision, it will be legal. Exxon or any other firm could spend Bloomberg-level sums in any congressional district in the country against, say, any congressman who supports climate change legislation, or health care, etc.
A century of campaign finance law was thrown in the trash today and that is going to be the result. These corporations have virtually unlimited money to spend on this --- ad campaigns are chump change by their standards and well worth every penny if they deliver politicians who will represent them in the congress. The deal is now explicit and it will be very difficult to unseat them if their elections are financed entirely by special interests.
From what I gather, there are only a couple of things to be done about this: shareholder empowerment or constitutional amendment, both of which are very, very difficult.
Update: Republican strategist John Feehery on Ed Shultz just said that Move-On raised so much money in the last election that this ruling will level the playing field.
Plus, the little guy is employed by corporations so they can be confident that they'll be represented by them.