Milton V. Peterson, chairman of the Peterson Cos., the developer of National Harbor. (Astrid Riecken for The Washington Post)

The contribution by the Peterson Cos. is the first to a leading pro-gambling group other than those from MGM Resorts, which is angling to operate a National Harbor casino, according to a group spokeswoman.

MGM has previously reported contributions to the pro-gambling group, which bills itself as “For Maryland Jobs and Schools,” totaling about $5.4 million.

Companies on both sides of what will appear as Question 7 on Maryland’s ballot have anteed up considerable sums in the first weeks of a campaign that got underway last month with the legislature’s authorization of an expanded gambling plan.

The money is funding an escalating ad war on television.

Penn National Gaming has contributed $5.5 million to the campaign to defeat the plan, which would allow Las Vegas-style table games at Maryland’s five other casinos as well as a new facility in Prince George’s County.

As of Tuesday, no other companies had reported contributions to the anti-expansion group, known as “Get the Facts — Vote No on 7.”

The pro-gambling group has also touted support in recent days from labor unions and other business groups, including the Prince George’s Chamber of Commerce, that have not yet contributed financially.

“We believe Peterson’s investment in Vote for 7 is particularly important given their firsthand experience with the job growth and economic development that a project of this magnitude can bring to the region,” said Kristen Hawn, a spokeswoman for the pro-gambling group.