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.