The first-term Lake Barrington Republican resigned from the House of Representatives last week. Politico reported that the Chicago woman claims Sauer posted explicit photos of her on a social media site to lure other men into sexual conversations.
Lake County GOP officials will decide on a replacement for the 35-year-old Sauer.
The woman told Politico she met Sauer online. They began a relationship and she shared intimate photos with him. The two broke up last spring. The woman says she filed a complaint with Chicago police in July when she learned Sauer had posted the photos.
Lake County State's Attorney's officials said Monday they're also investigating.