Maryland's gambling fight has shattered the record for most expensive political battle in the state's history as corporate spending passed the $40 million mark.

MGM Resorts International, which would operate a planned casino at National Harbor if the gambling expansion passes, added $3 million to bring its total spending up to $14.4 million. Caesars Entertainment, which is set to operate a new Baltimore casino, has chipped in $3.4 million, while the National Harbor developer has contributed $1.25 million in support of the expansion.

Penn National Gaming, which operates casinos in West Virginia and Perryville, raised its spending from $18.1 million to $21.6 million to defeat the measure, which would allow table games and round-the-clock operation at in-state casinos, in addition to adding a new facility in Prince George's County.

The previous spending record was held by the 2006 governor's race, in which Democrat Martin O'Malley unseated Republican Bob Ehrlich. That race cost about $34 million for the two sides combined.

Steven Taylor, a professor of urban politics at American University, said the constant spending increases from both sides indicate how close the fight seems to be.

"Corporations do not spend a lot of money on measures they think will fail," he said. "It's an investment for them -- an investment with the potential for high yields."

As gambling companies ratchet up spending, the Maryland Public Policy Institute entered the fray Wednesday when it released an independent study saying the potential economic benefits of the gambling expansion have been overblown.

"Some of the claims are grossly exaggerated," said Christopher Summers, the institute's president. "The other side says this'll put Maryland on the map, this'll create all these jobs -- but when you really put it under the microscope, the devil's in the details."

According to the study, about 45 percent of the revenues generated by a new casino in Prince George's would come at the expense of other casinos in the state. The estimated $424 million in gross revenues from the new casino would be partially offset by a gross revenue decline of $125 million in Anne Arundel County and $65 million in Baltimore, the study said.

Summers said approving expansion was not a long-term solution to Maryland's budget woes. "The state is up against a wall," he said. "They look at it as sort of a quick fix."

Kristen Hawn, a spokeswoman for the committee backing the casino expansion, said her group would respond once the study has been reviewed.