From Chamber of Commerce lobbyist Bruce Josten, explaining why they keep the names of their donors secret:
Corporations, as I said, have employees, vendors, suppliers, and shareholders of all political stripes. They’re not trying to alienate anybody. They’re looking for representative organizations, such as mine and thousands of others, to be an express organization to advocate for them on their behalf.
Whatever else you can say about the flap over the Chamber's funding sources, this is a notably unpersuasive argument. Josten is essentially saying that rich corporations want the ability to hound and attack anyone in the political sphere they don't like, but want to be protected from being hounded and attacked by others. That's nice work if you can get it, but I don't think most Americans will be sympathetic. If you want to be in the arena, then you need to be in the arena. Being a corporation doesn't — and shouldn't — endow you with a special exemption from being attacked if you take controversial political views.
UPDATE: Chamber CEO Tom Donohue, as usual, puts things more bluntly: "I want to give them all the deniability they need," he says. And he does.