They stop giving you money, spend it on helping your opponent win. Manipulate congress to impeach you. Also they can blackmail you since you technically took a bribe, you just didn’t honor your end of the deal. Realistically, it wouldn’t be the rich person himself/herself talking to you about it, but someone that works for them. They can have one of their goons admit bribing you and they can exchange their testimoney against you for their immunity against prosecution. They might’ve made a secret recording of the conversation, but conviently left out the part of which wealthy billionaire they’re working for. You might not even know the identity of which billionaire, all of the interaction could be through messengers.