MGM Resorts has opened an office at National Harbor, where it has proposed a new casino. (Jeffrey MacMillan for The Washington Post)

With its latest contribution, MGM has now spent $11.4 million on the referendum effort, and total donations from companies on both sides exceed $27 million. The money is fueling an extensive ad war.

At issue is Question 7, which would allow a Las Vegas-style casino in Prince George’s County, as well as table games at the state’s five previously authorized slots sites.

A group of business leaders, including the chairman of the Prince George’s Chamber of Commerce, is holding a rally at National Harbor on Thursday morning to support the plan.

The rally has been organized by For Maryland Jobs & Schools, the ballot-issue committee that has taken contributions from MGM and other companies to support the expansion plan.

The developer of National Harbor has also given $400,000 to the effort, and a group led by Caesars Entertainment has kicked in $2.3 million. Caesars is planning to open a Harrah’s-brand casino in downtown Baltimore in 2014.

The committee also announced Thursday that nine other businesses will contribute to the pro-expansion effort. They include several with a stake in the success of National Harbor, including Wyndham, which has a condominium-style hotel on the property.

Penn National Gaming has given $13 million so far to help defeat the ballot measure.

Penn owns Rosecroft Raceway, a Prince George’s facility that would be eligible to bid for a casino license if the expansion plan passes. But Penn officials argue that the deck is stacked against them, because Prince George’s County Executive Rushern L. Baker III (D) and others have championed National Harbor as a casino site.

Penn also owns a casino in Charles Town, W.Va., that analysts say would take a significant hit if another large-scale venue opens in Maryland.

This post has been updated since it was first published.