Establishing a wealth tax based on the rolling 10 prior years worth of PAC donations would feel somewhat fitting. A basis point per $50k or something.
To give a million to any party you have to pay 2 in tax.
All of that money should go to lower funded candidates so spending above the limit becomes self defeating. Republican mega donors might end up supporting a small communist party more than their own.
Musk dumping $360M into a political race is trivial to him. Making it cost three times as much to make the donation isn't a huge factor.
Making contributions have an impact on the donors total wealth will democratize these massive campaign spends.
I don't know why this wouldn't be reasonable for the USA.
However in 2010 the Supreme Court ruled in Citizens United that corporations (including so-called Political Action Committees) and unions cants have their expenditures capped. A subsequent decision in SpeechNow.org ruled that contributions to these PACs can't be capped, as long as the PAC doesn't coordinate with a candidate.
So here we are. I'm not sure if taxation on these types of contributions would be considered limitation of free speech, but I'm willing to give it a shot in courts.