One way to attack campaign finance is to argue it results in corruption. This is an argument about outputs, and it ends up being a complex and slippery argument to prove or measure. An alternate way is to take an “equality of inputs” approach:
Equality of inputs is a measurable standard, particularly if we measure it in terms of money contributed. By contrast, corruption is ultimately impossible to prove because it requires agreement on what the political system would produce absent some said corruption. If you can find me a universal definition of the public interest measured in policy outcomes, please send it. I’ve never seen one. All I’ve seen is vehement disagreement over the public interest.
Equality of inputs is an interesting approach to some issues in education as well.