I can't find the link now, but something that really opened my eyes on terrifying PACs are was someone pointing out how they manipulate the political discourse without even spending a dime. They just sit there on a gigantic warchest and send out gangster-style missives like "hey it sure would be a shame if you supported X or Y and we had to drop $10M on pummeling you with primetime TV ads in the week before your reelection."
This kind of influence is completely untraceable through all the normal disclosure mechanisms since no money actually gets spent. And worse still, it offers enormous leverage— once you have that war chest, you can use it to issue hundreds of threats and only occasionally have to actually follow through.
It's just a completely different ballgame compared to a "normal" campaign finance option that directly spends donations.
Are you sure? Pretty sure they can. Either way they can donate huge amounts to PACs, create their own PACs, run ads, etc. Bribery is pretty much legalised, it's ridiculous. See Citizens United v. FEC for example.
The whole point of a PAC is that they don't donate to the campaigns directly, but rather exercise their free speech to supply the public with "information" about the candidates.
