Advertisements opposing an expansion of gambling in Maryland are flooding the airwaves, as casino groups and labor organizations fight for their stake in the state's gambling industry.

Maryland residents, who will be asked to vote on an expansion of gambling in November if Gov. Martin O'Malley's gambling bill passes the General Assembly, will have a difficult time figuring out who is behind which ad, as tax-exempt nonprofit groups pool their funds to pay for the advertising campaigns on TV and radio.

The most likely source of funding is groups with a financial interest in the casino debate -- in this case, casino operators, developers and contractors with millions of dollars to gain or lose based on the governor's bill.

"Why else would you do it?" asked Trevor Parry-Giles, professor of political science at the University of Maryland. "These are not cheap things to do, and you've got to have a stake in the game. Nobody runs ads out of the good of their own heart."

Anti-casino ads have been aired by the Taxpayers Protection Alliance, a Virginia group that declined to reveal its donors, and the Prince George's County Contractors and Business Association, whose ads are paid for by the group's 500 members, according to Chairman Joe Gaskins.

Officials at the Cordish Cos., the owner of a new Anne Arundel County casino and the most vociferous opponent of a casino in Prince George's, said they haven't spent a "penny" on ads for either organization.

Gaskins, too, said none of his ads, including a mailer to Prince George's County voters, was paid for by Cordish.

Ads in favor of a Prince George's County casino have received help from the Peterson Cos., developer of National Harbor. Maryland Workers for National Harbor has aired television and radio spots in Baltimore, which have been paid for in part by the Peterson group.

A spokeswoman for the Peterson Cos. declined to specify how much the company has spent on ads. - Ben Giles