You could start with treason, or the Emoluments clause...
https://www.washingtonpost.com/politics/what-is-the-emoluments-clause-does-it-apply-to-president-trump/2017/01/23/12aa7808-e185-11e6-a547-5fb9411d332c_story.html?utm_term=.19edc75c41ed