> Edit: It's a common misconception that Citizens United allowed unlimited campaign contributions to candidates. That's not quite true--it allowed unlimited independent expenditures by corporations, but not unlimited contributions directly to candidates. The Supreme Court held in Buckley (1976) that the limit on direct contributions was a constitutional restriction on free speech because of the government's interest in "the prevention of corruption and the appearance of corruption spawned by the real or imagined coercive influence of large financial contributions on candidates' positions and on their actions if elected to office." That ruling still stands.

The trouble is that it has the same effect, and creates the same appearance of corruption. Acausal trade is possible; a candidate and their PAC do not have to directly communicate to end up influencing each other's behaviour.