I vaguely remember a post on GAW a few months ago of something quite similar to what Trump did with hush money and campaign funds, I've only found these articles so far.
https://www.politico.com/news/2020/07/31/sanford-bishop-allegedly-misused-campaign-funds-389742
Off the top of my head, you could research how congress has a slush fund paid for by taxpayers to pay off sexual harassment cases brought against congressmen. If paying off those kinds of claims now count as campaign contributions, then you've got months worth of digging to do.