Penn National Gaming, the owner of the first Maryland casino to open, will on Thursday formally announce its opposition to a November ballot measure on expanded gambling.

A scene from Rosecroft Raceway. (Photo by Bonnie Jo Mount/The Washington Post)

The legislation would authorize a new casino in Prince George’s County, as well as Las Vegas-style table games at Maryland’s five existing slots sites. It also allows existing casino owners to keep a larger share of slots proceeds as compensation for the competition a new casino will bring.

“We feel we have been left with no choice but to join with other concerned citizens and local business owners to educate Maryland citizens about the proposed law’s deficiencies, and to argue that Maryland would in fact generate more casino tax revenue for the state if the law were to remain the same, as opposed to an expansion plan that results in significant tax breaks for casino operators,” the ad says.

Penn owns Hollywood Casino Perryville, which opened in Cecil County in 2010, as well as Rosecroft Raceway, a harness track in Prince George’s.

Under the recently passed legislation, Rosecroft would be among the sites eligible to bid for a Prince George’s casino license. But Penn argues the deck is stacked for another location in the county: National Harbor.

The letter also seeks to rebut frequent criticism that Penn’s primary aim is to protect a casino it owns in Charles Town, W.Va.

“If that were the case, then we never would have offered to invest $500 million in a new establishment at Rosecroft,” the ad says.

MGM Resorts, which has been lined up to run a casino at National Harbor, has taken the lead among proponents of the gambling plan. The company so far has donated $2.4 million to a ballot issue committee supporting the new gambling law.