There already sort of is one in the US for candidates who exceed a certain threshold of the vote. It comes with spending limits that could be exceeded by a lot by raising money outside it, so parties stopped taking it. Plus tax payers have to opt into doing it, and the rates of people doing so have dropped over time

If you’ve ever filed US income taxes and seen the question

“Do you want $3 of your federal tax to go to the Presidential Election Campaign Fund”

That’s what it’s referring to