Powerful House Speaker Michael Madigan says he will support legislation to put up the money as a "safety-net" in case the Games come here and lose money.
The assurance was made in an April Fifth letter from Madigan to the city's chief Olympics organizer, businessman Patrick Ryan.
Governor Rod Blagojevich sent a similar letter in March.
The U-S Olympic Committee meets Saturday to decide if Chicago or Los Angeles will be the American bid city.
California Governor Arnold Schwarzenegger already has signed legislation authorizing 250 million dollars in state money for the Games if Los Angeles is chosen.