Re: They should just look into..
I mean... one simple case would be that it is illegal for a sitting president to use his office to endorse a private business, product, or service.
https://thehill.com/homenews/media/398685-trump-endorses-jeanine-pirros-new-book-in-oval-office
Literally, using his Office to promote someone's book. It would be a bit like convicting Capone for tax evasion.