What we are/should be discussing are the rights of the individual voters. The OP and subsequent conversation has shown that large contributions from corporations, unions and PAC's can offset any potential financial voice that I may have by overshadowing it. My case is that the individuals rights are being endangered by this, you seem to be coming at it from the opposite direction.
I an an AARP member, and as such the AARP has a very loud and distinct financial voice. I don't agree lockstep with every single decision they make, same as many NRA members don't agree lockstep with them, but in order currently to have any financial voice I have no other alternative.
If entities like this were prohibited from participating from the process then my small financial voice could possibly carry much more weight.
Again, this should be about the rights of individuals being swept away by things listed in the OP.