It will make it a CRIME to contribute more than that. It will become easier to sort out who is breaking the law and who is not. Corporations and billionaires will have no choice but to break the law and be sneaky if they want to have more influence. And if there is a red flag system in place, consumers can decide what corporations they dont want to do business with. That will also influence corporations to behave.
For instance, if I see indications that Coca Cola MAY be contributing way more than the legal amount, then I can choose not to buy it anymore. And believe me, I WOULD make that choice and stick with it until the red flag by that company's name disappeared. We as consumers HAVE to learn to play hardball like that.