Gov. Pat Quinn launched Illinois' epic attempt to bail itself out of a $100 billion public employee pension debt Thursday, signing into law an ambitious financial, legal and political effort to restore the state's tumbling credit ratings and unstable economy.

Despite the historic nature of the law, which takes effect June 1, the Democratic governor signed the measure behind closed doors, joined by top lawmakers. The quietness of the event symbolized the controversial nature of a package that has that split longtime political allegiances and quickly become fodder for the 2014 campaign season.

The private bill-signing stood in sharp contrast to the public pep rally that master of ceremonies Quinn held when he put his signature on Illinois' gay marriage law little more than two weeks earlier, a move that reaffirmed support among liberals.

On Thursday, Quinn offered a simple signing statement on the pension measure saying "Illinois is moving forward" and calling the law "a serious solution to the most dire fiscal challenge of our time."

Whether the law represents a solution to the state's pension liability will likely be up to the courts to decide. As expected, public employee unions said they had directed their lawyers to prepare a lawsuit to challenge the constitutionality of the law and seek a stay to prevent it from taking effect.

The We Are One coalition of unions, headed by the state AFL-CIO, called the new law an illegal "attempted pension theft."

"Leading politicians and their followers chose to violate their oaths of office, trample on the Illinois Constitution, and willfully ignore the plain letter of the law," the coalition said in a statement. "Once overturned, its purported savings will evaporate and the state's finances and pension systems will be left in worse shape."

At issue is a clause in the 1970 state constitution that defines public pensions as an "enforceable contract" with benefits that cannot be diminished or impaired. But supporters of the measure, including House Speaker Michael Madigan, have maintained that the law will be upheld by the Illinois Supreme Court.