Expanding gambling in Illinois is no sure thing.

Legislation authorizing a dramatic expansion of gambling has been approved by the General Assembly, but Gov. Pat Quinn says he's undecided about whether he'll sign it into law or veto it.

Meanwhile, there's continuing talk among legislators about trying to address some of Quinn's concerns.

That's the political side of the issue, and it's huge. But the issue of state-authorized gambling in Illinois is really about revenue, and the picture there isn't as bright as many legislators hope and gambling advocates contend.

A report issued by the Illinois Department of Revenue said gambling revenues increased from $60 million to $126 million during the first two months of the fiscal year, but still lagged more than $30 million behind what was forecast.

Authorities attributed part of the revenue decline to a lagging economy as well as to the legislature's decision to ban smoking at Illinois casinos.

Yet another reason is that the new Rivers Casino in Des Plaines is taking business away from other Illinois casinos.

The Revenue Department report said state officials predicted a 12.5 percent cannibalization rate from the Elgin Grand Victoria Casino and the Aurora Hollywood Casino, but reported that "revenue has not declined nearly as much as expected" at Aurora, while "Elgin revenues have declined twice as much as expected."

The report indicated that the state did not anticipate any effect on Joliet's two casinos by Rivers Casino because they are "separated by over 40 miles," but that "initial indications are that Rivers may be negatively impacting them as well."

These are unintended consequences, but consequences nonetheless, and they do not bode well for the predictions of those forecasting a tax windfall and creation of hundreds of jobs in communities that would get a new casino under the bill Quinn is considering.

Supporters of a Danville casino are counting on more jobs and increased local tax revenue if they get their casino. But even if they get it, they'd be well advised to temper their expectations because gambling-expansion advocates routinely over-promise and under-deliver.

Gambling in Illinois initially was promoted years ago as a way to draw tourists. But studies have shown that it doesn't. A casino in Chicago would be a definite draw for visitors to that city, but it also would have a huge negative effect on the four Chicago-area casinos.

A state legislative committee that studied the issue of gambling expansion recently concluded that, absent some new marketing success, "the casino industry will continue to struggle to improve in the years ahead, especially at the older casinos."

Those warnings don't have much effect on gambling expansion enthusiasts. But given the past, it seems pretty clear that the many promises surrounding gambling expansion will fall short.