Of course, once your donor calls you for a favor, you know you want their money for your next campaign, so you’re going to give them what they ask for. If you don’t, they won’t donate any more money to you. So the only thing that makes it not bribery is that you haven’t made a formal agreement to do the exchange of services for money. It’s just “understood” between the parties.
That is why I have advocated for requiring all political donations to be ANONYMOUS. You can't sell influence or favors if you don't know who is buying. Americans should be able to donate any amount they wish to any candidate, officeholder or party; that's FREEDOM. But purchasing an outcome makes it bribery and a criminal act.
Will they try to cheat? Sure they will, and they should be arrested and charged with a felony. Some have said in response that because they'll cheat, we should not do it. That's like saying because some invaders will try to tunnel under the wall, we should not build one.