For the casino giants trying to persuade Marylanders to vote for or against more gambling here, the long-term stakes could be in the billions.

Penn National Gaming's investment of $13 million for ads urging a "no" vote on Question 7 is pocket change compared with the revenue it stands to lose at its Charles Town, W.Va., casino if Maryland expands gambling. One analyst estimates the West Virginia site accounts for $200 million a year in cash flow for the company — a quarter of its total.

And the $11 million that MGM Resorts International has spent so far to win voter approval of a new gambling site in Prince George's County is a small bet compared with the return the company stands to gain from its proposed National Harbor casino. Consultants predict the venue would bring in at least $424 million in revenue a year.

"The stakes are clearly very high to justify the investment in the ad campaign," said James Karmel, a Harford Community College professor who tracks the gambling industry. "This is a really big deal for both of these companies."

The Central Maryland market is so lucrative that Penn National was reported this week to be in talks with the Cordish Cos. about buying a stake in the Maryland Live Casino at Arundel Mills — which took in nearly $32 million in revenue in just its first three months of operation, according to its reports to the state.

The focus of attention is the Maryland ballot question which, if approved, would authorize a sixth casino in the state — to be located in Prince George's County — and the addition of table games at all six.

Also in the spending fray is a Caesars Entertainment affiliate that holds the license to open a casino in downtown Baltimore. The MGM-backed ballot committee, dubbed FOR Maryland Jobs and Schools, has spent about $2 million from the Caesars group.

The Caesars group stands to gain $38 million a year if it is permitted to offer tables games such as poker, blackjack and roulette in addition to the currently permitted slot machines, according to a forecast for the General Assembly.

There are other companies with a stake in the outcome of the referendum. Cordish will certainly be affected but so far has chosen to sit out the ballot fight. And Peterson Cos., developer of National Harbor, could see the value of its property increase if voters say yes to a casino there.

But no company has more at risk than Penn National, the Wyomissing, Pa.-based owner of the Hollywood Casino in Perryville, Md., and Rosecroft Raceway in Prince George's County in addition to its West Virginia property.

Vote No on 7, the advocacy group financed by Penn National to fight the ballot proposition, has been spreading the word that the gambling expansion law passed by the General Assembly last month is a bad deal for taxpayers that will not necessarily deliver its promised benefits for education.

Whatever the merits of those arguments, gambling industry analysts say Penn National's interest in the referendum is primarily about protecting its business at Charles Town.

The Hollywood Casino at Charles Town Races is Penn National's largest property. With 209,508 square feet, it is larger than the company's three Mississippi casinos put together. According to Penn National's annual report, Charles Town offers 4,180 slot machines — more than any casino in Las Vegas — along with 107 table games, 30 poker tables, restaurants and entertainment. Its parking lot has space for 6,048 vehicles, and at any time a hefty percentage of them sport Maryland tags.

"It's the crown jewel right now, absolutely, hands down," said Chad Beynon, an analyst with Macquarie Capital in New York.

It's a jewel that could lose a lot of its luster if a new, glitzy "destination resort" casino opens at National Harbor, analysts say.

Beynon estimated that Charles Town could account for as much as one-quarter of Penn National's annual cash flow. A casino at National Harbor, right off the Capital Beltway instead of an hour's drive from Washington, could take away 25 percent of the West Virginia casino's business, he said.

Penn National posted $2.7 billion in revenue last year.

Karmel said the opening of a National Harbor casino — coming on top of new competition from Maryland Live and the Baltimore casino expected to open in 2014 — could leave Charles Town in a position like Atlantic City's struggling casinos after the Philadelphia market opened to gambling.

"Penn National has reason to be concerned about a big casino right smack in the middle of the D.C. market," Karmel said. "They've pumped a lot of money into [Charles Town]. It's been one of their premier properties for a few years now."

Approval of the referendum would not just bring new casino competition. If the measure passes, Maryland casinos will also get table games, eliminating an advantage Charles Town now holds over Maryland Live.