Maybe donations to political parties could be just taxed at 200% (technically ~76% but 200 sounds better)?
To give a million to any party you have to pay 2 in tax.
That would make it more expensive but still worth it for a billionaire who really cares about their favorite getting elected.
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.
Even better, tax progressively. 0% under $100, 200% above $1000.