> "We'll spend $X on advertising for your campaign if you vote for this bill"
That's illegal.
The advertising is given no strings attached. The only incentive that you (the politician) have to vote for things that favor the company is that, if you don't, they may not advertise for you next time.
The "fun" part of this whole thing is that the money those companies spend isn't really the issue, it's the fact that the money is helpful to influence people. Comparatively, if a person of non-political celebrity tweets that they like a candidate, they might muster much more influence than say, Exxon could for the same candidate. Now that candidate is just as beholden to the interests of that celebrity as one might be to a corporation.
In effect, the PAC money really is just taking money away from influential individuals and commoditizing it.
I, personally, don't really see a difference. It's not like pre-Citizens United America was some sort of paradise. Most people perceive it as worse but I just see it as different. I, personally, still have a similar amount of political power and influence.