Casino owners upped the ante this week in a high-stakes ad war over the expansion of gambling in Maryland, bringing total spending on both sides of the issue to a staggering $18 million.

Las Vegas casino giant MGM Resorts International dumped $3 million into the fight on Monday, bringing its total contributions to $8.4 million since late August, according to a report filed with the Maryland State Board of Elections on Tuesday. MGM has a deal with the developer of National Harbor to build the state's sixth casino there if voters approve the gambling expansion, which also would allow table games and round-the-clock operation at all casinos.

On the other side of the issue, casino owner Penn National Gaming has forked over $9.5 million for ads protesting the Prince George's County casino. Penn National is worried that an $800 million casino on the Potomac River would pull business away from its casinos in Perryville, Md., and Charles Town, W.Va.

The total already surpasses spending in Maryland's 2010 governor's race between Gov. Martin O'Malley and former Gov. Bob Ehrlich, which added up to $17 million between the two candidates. With seven weeks until voters decide the issue at the ballot box, heavy spending on both sides is expected to continue -- and residents will continue to be pummeled by TV and radio advertising blanketing the Washington and Baltimore areas.

"There is going to be more money spent on this issue than any other issue on the ballot," said Todd Eberly, a political scientist at St. Mary's College of Maryland. Also on the ballot this year are questions of whether to legalize same-sex marriage, whether to permit some illegal immigrants to pay in-state tuition rates and a challenge to the state's congressional redistricting.

"Most of the money is going to be spent by the folks who want to expand gaming because they want the revenue from expanded gaming," Eberly said. "It should tell people a little bit about how much they expect to make off of this deal, since in a very short amount of time, they are willing to bet that many millions of dollars on the outcome."

Information on spending for other ballot measures will not be known until after the election. But the General Assembly required early disclosure of spending details on the gambling question during its four-day special session in August -- when gambling interests spent roughly $3.6 million on lobbying, according to reports filed with the Maryland State Ethics Commission.

The biggest spender on lobbying was the Washington DC Building and Construction Trades Council, an organization of labor unions, which spent $2.7 million on television ads backing the gambling expansion, according to the disclosure reports.

Maryland Comptroller Peter Franchot said the sheer magnitude of the lobbying expenditures shows that Maryland needs more openness in its campaign finance reporting system.

"If citizens had this information at their disposal during the special session, we could have had a more enlightened public debate and perhaps a different outcome," he said. Calling for a more up-to-date reporting system for campaign finance disclosures, he added: "This has nothing to do with technological capability and everything to do with political will."