With its dress code, high beamed ceiling, and expansive window of century-old leaded glass, the Campbell Apartment is a strange place indeed for a bar brawl.

But a winner-take-all war has raged for six months within the granite walls of this dark and elegant dowager of a cocktail lounge in Grand Central Terminal — a battle that pits old against new, staid against trendy, an Old Fashioned with a stirring rod against black barrel whiskey with muddled raspberries.

This week, old — in the personage of longtime owner Mark Grossich — waved the white flag.

“It sucks, it blows, it’s totally unfair,” Grossich said of losing his lease, after 17 years running Campbell Apartment and a half a year fighting eviction in court.

Originally the offices of a Jazz Age financier named John W. Campbell, the space had been a water-damaged, drop-ceilinged shell when Grossich, at a cost of $2.5 million, lovingly restored it to its original, baronial splendor.

As CEO of Hospitalty Holdings, Inc., which owns Midtown’s Carnegie club and Murray Hill’s Lexington’s, Grossich is a master of the timeless, intimate cocktail lounge, temples to single malt scotch, fine cigars and tufted upholstery.

The Campbell Apartment was a cornerstone of that civilized empire.

But in one month, on July 28, Grossich must turn over the Campbell Apartment space to its new lease-holder, Scott Gerber, who runs such hip, jangly and galvanic lounges as the Irvington in the W Union Square and Mr. Purple on the roof of the Hotel Indigo on the Lower East Side.

The new guy plans to transform the Campbell Apartment and its “business casual” dress code into something far less stuffy. Something less Brooks Brothers, more limited edition sneakers and Gucci T-shirts.

“Right now, the image that people have of it very often is it’s a place to go before special occasions,” Gerber explains.

“So if you’re going to a black-tie event at the Hyatt in Grand Central, you go in [to the Campbell Apartment] for a drink. That’s OK, you can be in a tuxedo,” Gerber says.

“But it’s also OK for a guy or a girl to be in jeans or a T-shirt. That’s OK too. That’s the way people live,” Gerber says, reasonably enough.

“We have a lot of athletes or musicians or celebrities who come into our places — what are you going to do, tell them we don’t want you in because you’re wearing a denim jacket?”

Well, yes, tell them exactly that, Grossich would counter.

Plenty of celebs come to Campbell Apartment as well — George Clooney, Liam Neeson, President Bill Clinton among them — and none of them wore sneakers.

“Trendiness,” he scowls of Gerber’s plans. “I hate that word.”

Nearly a hundred years before it became a bit of an existential football, the Campbell Apartment was the business and pleasure lair of a millionaire financier named John W. Campbell.

A close pal and associate of William Vanderbilt — then chair of the New York Central Railroad — Campbell acquired the lease for the 25-by-60-foot space in 1923.

It was near Campbell and his wife’s Park Avenue home, and so would be a convenient spot for running his credit-reporting business by day and entertaining clients by night.

Campbell really knew how to trick out a corner office.

He installed a grand piano, a pipe organ, a faux stone fireplace featuring his Scottish family’s coat of arms, a 30-foot ceiling with faux-wood plaster of Paris beams, and a massive hand-knotted Persian carpet, one of the largest in the world, that would have cost $3.5 million in today’s dollars.

Campbell, who died in 1957, was more a man of numbers than letters, and never ascended to the Astor 400, so there is scant record of him in libraries or old society page clippings.

But Grossich has spoken with his niece, Elsie Fater, who remembers that Uncle John oddly eschewed socks, even while wearing shoes.

‘He sat there at his desk in his underpants, so as not to lose his crease.’ - Author Allyn Freeman on John W. Campbell

Nor would he suffer wrinkles in his trousers, preferring to sit at his massive desk in his skivvies until an appointment required otherwise.

“He sat there at his desk in his underpants, so as not to lose his crease,” as Manhattan author Allyn Freeman, the closest thing to a Campbell biographer, put it recently.

Freeman’s brief collaboration with Grossich on a biography was abandoned when extensive research turned up “almost nothing,” he said, though a black and white photo of the man and his office does survive.

“You go through the door and the armoire is on the left, and there’s a big rug that greets you with a table and a flower arrangement,” Freeman said, describing the photo.

“There’s a couple of tufted chairs to the right, and at the far end behind this 15th-century Florentine desk is John Campbell.”

When the tycoon died, “The furniture went no one knows where,” Freeman said.

“CBS Radio did some broadcasting in there, and then it dilapidated,” he said.

“At one point, the police put in a pokey, a cell, for all the wastrels and drifters that came through Grand Central.”

By the time Grossich acquired the lease, “There was water damage, wires hanging down, all the colors had faded, every piece of furniture was gone.

“But he had the vision and the money to redo it.”

Traverse a seemingly private passageway, then climb a tucked-away carpeted staircase to find the Campbell Apartment, a dark and stately bar so hidden away in the bustling crypt of Grand Central Terminal that even time seems to have abandoned it there.

Big Band standards waft softly from hidden speakers. Wait staff in their uniforms — white dinner jackets and black ties for the waiters, black cocktail dresses and double strands of pearls for the waitresses — carry Delmonicos and potent Prohibition punches across slightly threadbare carpeting.

“Cocktails from another era,” the Campbell Apartment menu promises.

“There’s a certain timelessness to the space that makes it so interesting,” Grossich says.

‘Cocktails from another era,’ the Campbell Apartment menu promises.

Grossich took the lease to the space in 1999, and spent nearly two years and more than $2 million on the renovation.

“It was an absolute dump,” he remembers. “Half the pane glass in the window was broken, doors were broken. Nothing you see was visible.”

Painters had to lie on their backs on scaffolds for months to restore and paint the colorful, heraldic designs on the high ceiling beams.

The result, Freeman says, was lovely.

“It was a terrific place to meet,” says Freeman.

“I took people there and they just exclaimed how wonderful and how different it was.”

Grossich signed a five-year lease when the 10-year lease ran out, and month-to-month leases since then.

But last year, the MTA began aggressively overhauling Grand Central’s restaurants and bars, hoping for higher rents and ever-more high-end lease holders.

By early this year, Junior’s and Two Boots Pizza lost their spots, and Cipriani Dolci was waiting to hear if it could stay.

New leases were awarded to an Apple store and a Nordic food hall operated by prominent Copenhagen restaurateur Claus Meyer. Wichcraft, the sandwich chain owned by celebrity chef Tom Colicchio, is taking over the space Junior’s had for 16 years.

Meanwhile, without Grossich knowing, The Gerber Group was approached by the MTA’s hired advisers at Newmark Grubb Knight Frank, inviting a bid on the Campbell Apartment space.

“I said, ‘I’m really not interested,’” Gerber remembered. “And they said, ‘Look, we think you’d be great, you’re the kind of person we want.’ And I said, OK.”

In late December, Grossich, who’d been paying $350,000 a year on his year-to-year leases, learned that his offer to pay $800,000 a year on a new 10-year lease had been outbid.

Gerber had offered $1.1 million. As lease holder, Grossich was entitled to make a best and final offer.

“I told them I’d pay whatever the highest bid was, plus 2.5 percent,” said Grossich.

“Oh no,” he says he was told. “They said, ‘They way overbid you. We can’t do that.’ Now after 17 years and all we’ve done to help rebuild the terminal, you can’t do that? Are you kidding me?”

In April, Grossich sued the MTA, successfully barring them from transferring the lease, but only for a few months.

Gerber wants to run a “sleek and glitzy” club, Grossich argued in his lawsuit, not the kind of dignified restaurant and bar the MTA sought in its own bidding paperwork.

Gerber countered they had the better proposal and the higher rent offer, and that unless they could begin renovations immediately, they’d lose hundreds of thousands of dollars in holiday party bookings.

After many back-and-forths in Manhattan Supreme Court, a judge last week declined to extend Grossich’s stay of eviction.

On Wednesday, the MTA gave Grossich his 30-day eviction notice.

“It’s a sad turn of events,” Grossich said of the eviction.

He wishes his successors luck — they’ll need it, he said.

“It’s going to be very difficult to achieve the kinds of numbers that will be necessary to sustain the operation for the long haul,” given the million-plus rent, he said.

Gerber, meanwhile, is eager to start the refurbishment.

It’s a landmarked space, meaning he can’t change the walls, ceiling or century-old windows. But he’s installing a costly new stone bar top, new bar and kitchen equipment, a new heating and air conditioning system, he says.

The club chairs and faded carpeting are bound for the trash heap, and instead of the fringe-shade wall lamps, Gerber says he’s bringing in multiple chandeliers and high-tech lighting to better show off the beautiful ceiling.

Frank Sinatra, too, may have gone the way of John W. Campbell, and softly crooned his last. Instead, there will be “eclectic” music for an “eclectic” crowd, Gerber promises.

“I just think it needs to be — today.”