Expecting Washington to determine who is a serious candidate (and then policing their use of federal funds) is a major conflict of interest, because policymakers will almost certainly set standards that benefit the two entrenched parties. I've toyed with an alternative concept, though, that might achieve the same effect with less required oversight.

The idea is a sort of non-partisan "un-PAC" -- a fundraising organization explicitly dedicated to combating corporate political donations. If a presidential candidate or his affiliated PACs recieved eg. $5MM in fundraising from oil executives, the un-PAC would spend $5MM on negative advertising linking him in the public's mind with that industry. If the un-PAC raised enough money, eventually a large multiple of corporate donations could be spent counteracting them, and at some point it would become uneconomical for politicians to accept those donations at all.

We spent about $6B on the 2016 federal elections (https://www.opensecrets.org/overview/cost.php). That comes out to about $20 for every American. That's a big number to beat, but not impossible, and every bit along the way helps.