I would completely support an ammedment like this. However I can think of a couple issues american citizens would have with it.
*Reduces political power* Americans like to believe they can have an effect on the system. By making a donation to a candidate it lets you know you are having an impact on the election. One of the last things we need right now is for people to feel like their opinion does not matter. A large part of the population does not vote and doing somehting like this may give a very cynical attitude towards for americans.
*May reduce parties vying for smaller states* Political parties always tend to go for the bigger states for more votes int he electoral college, this would be almost exclusive if this were to pass as with such limited spending they would want to make their voice heard by the states with the biggest population and not try to teach out more to the smaller ones.
Keep in mind I do support the idea however if that is any consolation. I believe electoral candidates should have a set limit on their spending to stop corporate america and lobbyists from completely buying out a candidate. While I believe in capitalism and in the idea that your earn what is given to you I do not think they should have an overextended amount of power simply because their wallets run deeper than the average citizen.