The state treasurer is battling Republican Congressman Mark Kirk, and a number of recent polls show it's neck and neck.
This is a crucial contest that could determine whether the Democrats retain control of the Senate.
The Senate seat was formerly held by President Obama.
The first lady said no president can succeed by himself, and her husband needs Giannoulias in the Senate.
Both the president and the vice president have campaigned in Illinois for Giannoulias.