How do you determine the ‘opponent’? If I want to argue against racism which racists do you give the money to? Just any racist in the street? Some organised racist group? Which one? What if my obvious opponent denies they’re racist are they still entitled to the money?

Okay, but wouldn't you consider an ad campaign "political" if it was about voting that oil refineries can't dump run off into a water supply? Would that mean that the same folks running the anti-dumping, environmentally friendly campaign also have to donate money to the oil companies for their pro-dumping initiatives?

I'm not trying to be mean, just pointing out an unintended consequence of such a system.

Politicians have more than one opponent (except in USA I guess...)

What about systems that are not strictly with two sides?

I'm not convinced, but there's a possible setup: a fund. Entities donate to the fund, and then the money would be distributed among the candidates (either equally, among a smaller number of qualifying candidates, or for all candidates, proportional to e.g. a party's representatives in the legislative branch)

(This would do nothing about money buying influence — as long as the donors are known, their wishes would be carried out by at least the major parties)