Penn National Gaming has contributed nearly $5.5 million to an effort to defeat Maryland’s expanded gambling plan at the ballot box, signaling an expensive battle heading into November.

Developer Milt Peterson stands at the site where he wants to build a high-end casino at National Harbor. (Photo by Astrid Riecken for The Washington Post)

Question 7 on Maryland’s statewide ballot will ask voters if they approve of allowing a new Las Vegas-style casino in Prince George’s County and table games at Maryland’s five other authorized slots casinos.

As passed by the legislature, the plan would invite bids for a Prince George’s casino for a swath of the county that includes both National Harbor and Rosecroft Raceway, a harness track in Fort Washington owned by Penn National.

Penn has argued that the deck is stacked against them, however, because Prince George’s County Executive Rushern L. Baker III (D) has championed the National Harbor site, a 300-acre mini-city on the banks of the Potomac River.

The pro-expanded gambling group has accused Penn of acting to protect a large-scale casino the company owns in West Virginia, arguing business there could be threatened by another large-scale casino in Maryland. Penn has denied that as a rationale.

In addition to donating to a ballot issue committee opposed to the gambling plan — which has already started airing television ads — Penn has also registered to make so-called independent expenditures during the campaign over the ballot issue.