Madigan has declined publicly to discuss her plans.
"I have not made up my mind yet about what I'm going to do," she told reporters Wednesday.
A number of Republicans — State Sen. Bill Brady, Sen. Kirk Dillard, Treasurer Dan Rutherford and U.S. Rep. Aaron Schock among them— have said they're thinking about challenging Quinn, who says he's ready to seek re-election. Former White House chief of staff Bill Daley, a Democrat, said he's also mulling a primary challenge.
Simon gave no clues about why she decided to make her announcement now and with such few details.
But campaign and political experts said announcing early allows Simon to fundraise separately from Quinn.
The Democratic governor has seen his approval ratings dip over the past months and been scrutinized by Republicans and his own party for being unable to broker an agreement on a pension overhaul, his top priority for more than a year.
"If she carries any baggage, it is that she is lieutenant governor under a governor who has mixed reception across the state," said Alan Gitelson, a Loyola University political science professor
Simon offers name recognition, popularity as a downstate Democrat and wisely used her office to travel statewide, experts say. She is the daughter of the late U.S. Sen. Paul Simon, worked as an assistant state's attorney in Jackson County and taught law at Southern Illinois University in Carbondale.
"She has her sights set on being her own person," said Paul Green, a political science professor at Roosevelt University in Chicago.
Simon said she told Quinn her decision in December.
He said Wednesday that he did not consider it as an insult that she was leaving and did not persuade her to stay.
"She told me she had pretty much thought about this and had other ambitions so I accept that," he told reporters an unrelated event.