Nice short video (4 mins) explaining “how to” bribe a politician, going over the three laws that make it possible and when/how they came to be.

TLDR: You can only give $5k to a candidate, directly or via a group (i.e. PAC). But you can give as much money as you want to a group/company that is not directly affiliated with the candidate. That group (a Super PAC) can spend as much money as it wants to support the candidate in any way - ads, posters, pamphlets etc. Thus, bribery is legal in the US as long as it’s not direct.