There are indeed caps on contributions to the political campaigns of candidates.

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.

But this doesn't help with taxation of the rich. There are very few ways the rich can't easily dodge. Tying tax to something they actually spend money on and can't really hide it because of the scruitany on the spending of poliitcal parties would be one such additional way.