Are you aware that the Citizens United case was actually about a movie? It wasn't about handing someone a stack of cash. When I see perspectives like yours, I wonder what you would say is the right way to handle the question of whether someone can make a movie that portrays a candidate in a positive or negative light. It seems to be pretty clearly a matter of free speech (first amendment), so unless there's some other provision of the constitution that would override that, I don't see how it could be forbidden.

To the extent that a pretty big chunk of donations are used to fund very short movies (we call them ads) for or against candidates, I'm not sure how that can be distinguished. I could see how one would distinguish get-out-the-vote or other similar non-speech type activities, but those on the Left seem to not oppose such expenditures.