Citizens United.

https://en.m.wikipedia.org/wiki/Citizens_United_v._FEC

It's some Supreme Court interpretation of free speech plus legislation. I remember Al Franken talking about this a bit in the context of a law requiring disclosure that he supported but was not successful.

Looks like the dynamic hasn't changed much since he talked about it.

https://www.reuters.com/world/us/biden-make-remarks-tuesday-...