The move sets the stage for a potential showdown with lawmakers following the Nov. 6 election, when supporters of the bill have vowed to try to override a veto.
For months, Quinn has warned about what he views as shortcomings in the bill, saying the measure would not provide enough oversight of casino operators and other gambling interests. The Democratic governor also has said that any gambling expansion should set aside a proper amount of money for education.
In his veto message, Quinn said the state must not allow ethical shortcomings that allow "loopholes for mobsters."
The governor told lawmakers it is "critically important that any expansion of gaming in Illinois be undertaken thoughtfully and carefully."
"We have one opportunity to get it right," Quinn wrote, arguing the legislation "continues to fall well short of the standards of he people of Illinois."
"The most glaring deficiency ... is the absence of strict ethical standards and comprehensive regulatory oversight. Illinois should never settle for a gaming bill that includes loopholes for mobsters."
"Notably, this legislation lacks a ban on campaign money from gaming licensees and casino managers. We must prevent campaign contributions by gaming operators from infecting our political process."
Mayor Rahm Emanuel issued a statement saying it's time to stop the flow of jobs and money to northwest Indiana casinos.
"I spoke with the governor this morning, and we agreed it cannot take another 20 years of discussion to draft and pass a bill that will be signed into law. I will continue to work relentlessly with all parties to pass a bill that will allow a Chicago casino to be built and implemented responsibly.”The deadline to act on the proposal was today, or it would have become law automatically.
The bill lawmakers approved this spring calls for new casinos in Chicago, southern Cook County, Lake County, Rockford and Danville. It also would allow slot machines at horse racing tracks, which was a deal-breaker for the governor when lawmakers approved a similar measure last year. That bill never made it to Quinn’s desk after he threatened to veto it.
Sponsors say they are prepared to fight this time around and have spent the summer trying to gather the votes needed to override Quinn's veto. It would take a three-fifths vote in the House and Senate. That won't be easy, even during the lame-duck session following the election when lawmakers who won't return to office are more likely to cast controversial votes.
Powerful House Speaker Michael Madigan, D-Chicago, has predicted an override won't be successful, but supporters say that's just one opinion.