You could make it so that even the receiver of the donation doesn't know where the money comes from.

I heard this idea somewhere else recently, and I think it's brilliant. Instead of limiting sizes of political donations, just make it impossible to track who gave 'em. Oh sure, you can still tell your favourite politician "Hey, I just gave you ten thousand dollars" but they can't possibly check.

Closing a few obvious loopholes: you should only be able to donate in certain nice round numbers ($10, $20, $50, $100, ... $10,000, $20,000) to avoid situations where you can tell a politician "Hey, the $15433.33 donation is from me" and have them confirm it. And it should be fuzzy in time, too -- politicians shouldn't know when a donation came in with resolution greater than a week or so.

It's great, because people are still free to support their favourite politicians as much as they like, but with greatly decreased possibility for corruption.

Politicians will, of course, hate it, so it'll never happen.

They don't even have to know the amount of the transactions. The "campaign donation bureau" can only releases monthly balances.

It's like drugs and weapons: if you ban legal donations, you'll get a democracy funded by illegal donations.