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

The contribution is the latest volley in an escalating battle — playing out in costly TV ads — over whether to allow a sixth casino in the state, as well as Las Vegas-style games such as black jack and roulette.

MGM previously gave $2.4 million to a ballot issue committee coordinating the campaign seeking to win voter approval.

Penn National Gaming has led the fight against the expansion plan, contributing $5.5 million so far to a committee opposed to the statewide ballot measure.

The plan, which will appear as Question 7 on Maryland ballots, was authorized by the legislature in a special session last month.

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.

Penn also owns an existing Maryland venue, Hollywood Casino Perryville in Cecil County.