Here at Gettysburg Souvenir & Gifts, just a rifle shot from the Gettysburg battlefield visitor center, a sign seduces passersby with news of the treasures for sale inside: 350 different T-shirts! 75 souvenir mugs! 225 flags! 60 toy guns!

So maybe this isn’t the best place to get all weepy about protecting sacred historical sites from the contamination of commercialism.

Yet, if there’s irony in the air, it doesn’t deter John Neal and Van Zabava from voicing their complaints – complaints that have echoed over the rolling hills of this Pennsylvania farm country for several months now. And the complaints will only grow louder. You can bet on it.

“It’s just not right to commercialize the place more than it already is,” says Neal, of Springfield, Illinois. He and Zabava, of San Antonio, are Civil War re-enactors in town for their semiannual pilgrimage to the site of the most consequential battle in U.S. history.

“If they get their way,” Zabava adds, “then pretty soon more of the landscape disappears, the serenity of the place disappears, and all you’ve got is more cars and noise and confusion and the battlefield will be lost.”

The “they” Zabava has in mind is Chance Enterprises, a development firm fronted by one of Gettysburg’s most prominent citizens, David LeVan, a former chief executive of Conrail Inc. and owner of the town’s Harley-Davidson dealership.

In April, LeVan announced a proposal to build a casino with 3,000 slot machines and an attached spa-hotel-theater complex on a 42-acre parcel a little more than a mile from the battlefield’s federally designated boundary.

LeVan is joining a gold rush set off by the Pennsylvania legislature last year when it legalized slot-machine gambling. The legislation authorizes two slots casinos to be built in Philadelphia, one in Pittsburgh and at least two others in locations to be chosen by the Pennsylvania Gaming Control Board. The board will begin to consider LeVan’s plan this month.

As always happens when developers meet opposition, a clash of dubious economic numbers has ensued. LeVan says his casino complex will create 800 jobs and generate at least $10 million annually in local tax revenue.

Opponents say those figures are inflated. They cite economic studies that show gambling’s municipal benefits dwindle over time and often are canceled out by the need for new roads, sewers and social services.

Besides, says a report from the Civil War Preservation Trust, the economics of the Gettysburg battlefield are already pretty good.

According to the report, every year Gettysburg draws 1.6 million visitors who spend $121 million on lodging, food, and gifts. (That’s a lot of coffee mugs.) This economic activity in turn provides 2,600 jobs for the Gettysburg area, the report says.

Turning Gettysburg into a gambling mecca, opponents say, will actually hurt existing tourism. Visitors who come seeking an educational and “family-friendly” experience will be put off by the breezy licentiousness that trails legalized gambling like a stink.

Unfortunately, the numbers presented to prove this point are shaky: In an informal canvass of “heritage tourists” over the July 4 Independence Day weekend, 53 percent said they would be unlikely to return to Gettysburg if a casino were built.

It’s doubtful whether such an unscientific survey will sway the commission when it weighs LeVan’s application. Besides, the strongest case against bringing a casino to Gettysburg involves values and sentiments that numbers don’t capture.

Gettysburg began drawing visitors before the smoke cleared in July 1863; it has continued to draw them because of its (relatively) unspoiled nature. Even today you can stand in the crags of Devil’s Den or climb Little Round Top and see the land much as the soldiers saw it.

In fact, the gambling proposal comes at a time when Gettysburg is becoming more, not less, authentic – truer than it has been in years to the feel and atmosphere of the Civil War landscape.

A 1970s-era observation tower, rising 30 stories and corrupting sightlines for miles in every direction, was at last demolished in 2000. The Depression-era visitor center, thoughtlessly placed along Cemetery Ridge near the heart of the fighting, will soon be removed to a less sensitive location, returning the present site to its original topography.

Best of all, the Civil War Preservation Trust is expanding the amount of preserved land – most recently buying and demolishing a multi-acre car dealership adjacent to the park.

Rehabilitation like this will only enrich the visits of people who come to Gettysburg for the best of reasons – to pay tribute to heroes, and to learn history by imbibing it straight up. A casino will bring visitors for different reasons – and will thus attract a different kind of visitor.

On the porch of Gettysburg Souvenir and Gifts, John Neal pointed past the Comfort Inn toward the battlefield.

“You go out there now and look how respectful people are of the place,” he said. “You hardly ever see people leaving garbage behind. That’s because they understand what this place means.

“You get a casino, you’ll have people come here just to kill time between bets. They won’t have the same respect.”

His pal Zabava nodded solemnly.

“What’ll happen is,” he said, “some guy will lose 10 grand and come out here and throw himself off Little Round Top. And that’s not what Little Round Top is for.”

Mr. Ferguson is a columnist for Bloomberg News.