If Donald Trump manages to win the presidency, his wife Melania will be the first foreign-born first lady since John Quincy Adams’ spouse, Louisa.

She’ll also be the only one — as far as we know — who has posed nude.

Certainly the only one to pose nude on a bearskin rug on her then-boyfriend’s private jet, as Melania Knauss did in British GQ in 2000.

“We have incredible sex at least once a day,” Melania told Howard Stern that year. “Sometimes even more.” Trump boasted about how hot Melania looked in “a very small thong.”

Eat your heart out, Eleanor Roosevelt.

So who is this 5-foot-11, 35-24-35 model who would be first lady?

Born Melanija Knavs in Slovenia in 1970 when it was part of Yugoslavia, she’s the daughter of an Austrian man named Victor, who managed a chain of car and motorcycle dealerships, and a Slovenian woman, Amalija, who, by various accounts, was either a fashion designer or a garment factory worker.

“My mom was in the fashion business. I was 5 years old when I did my first catwalk and did commercials at 16,” Melania told the website Parenting a few years ago. “I went professional after my studies. My mom loved fashion. We loved to travel and go to Italy and Paris.”

But her fashion career wasn’t a foregone conclusion. Stane Jerko, an acclaimed photographer, discovered a 17-year-old Melania hanging out at a cultural center in Ljubljana, Slovenia, during a modeling contest.

It was 1987, and he asked the teen if he could take some pictures of her.

“Melania was a very quiet, thin girl who appeared not to have too much aspiration for being a model,” Jerko, now 77, tells The Post. “She struck me more like a bookworm. I never expected that she would become a world-famous beauty.”

Well-mannered and shy, the teen went on to study architecture and design at the University of Ljubljana. Jerko lost touch with her when she started traveling to Milan, where, at age 18, she signed on to an agency.

It was in Italy that Melania — who changed her surname from Knavs to the more Germanic-sounding Knauss — was set on the path to meet The Donald.

“I met Melania in a previous life,” says ID Models Management founder Paolo Zampolli, who befriended her when she was booking jobs in Paris and Milan. “I invited her to join my agency in the United States.”

He described the future Mrs. Trump, who moved to New York in 1996, as being determined and very professional when it came to her career.

She was a homebody, not a “party girl,” he told The Post in 2005.

“This is a woman who modeled for Camel cigarettes on a huge billboard in Times Square but stayed home all the time,” he said.

Today, he adds, “She was absolutely business-orientated. She took her career very seriously.

“She came to do a job. She didn’t come for any other reason. So she was doing what a model should be doing — go to the gym and go to work.”

According to her best friend at the time, Edit Molnar, Melania had a “laid-back” style.

“She was reserved,” the former model once said. “The first party she ever came to with me was the one she met Donald at.”

That would be a party hosted by Zampolli at the Kit Kat Club during New York Fashion Week in 1998. She was 28. He was 52.

It was love at first sight for Trump, but when the well-known womanizer tried to get Melania’s number, she refused, asking that he give her his instead.

“I am not a girl who will just give away the number to anybody,” she later told The New York Times.

According to Molnar, the model was initially “turned off” by the billionaire.

“Melania said, ‘He’s here with a woman. I am absolutely not giving him my number.’ She wouldn’t even consider it,” she recalled.

Instead, Donald gave her all of his numbers — business and home — and she called him when she returned from a modeling trip.

Donald was separated from his second wife, Marla Maples, and would be divorced by 1999. He was squiring socialites around town, but after Melania called, he was a one-model man.

In the early days of her romance with Donald, as her fame and modeling career soared, Melania brushed off suggestions she was a gold digger.

“The press could be sometimes very mean,” she told the Times. “But I think you can’t be with the person if it’s not love, if they don’t satisfy you.

“You can’t hug a beautiful apartment. You can’t hug an airplane. You can’t talk to them.”

Seven years later, they married in a lavish, much-publicized ceremony at his Mar-a-Lago resort in Florida’s Palm Beach. Her Christian Dior dress alone cost $100,000 and had a 13-foot train and 16-foot veil.

You may not be able to hug wealth, but Melania has certainly embraced it. Her Twitter feed is peppered with photos of their jet-set lifestyle — shots of New York and Washington, DC, taken from their private plane, selfies of her draped in jewels and fur.

Photos of the Trump Tower penthouse suite she shares with Donald and their son Barron, 9, give her 42,400 Twitter followers a glimpse into their opulent Manhattan lifestyle.

The Louis XIV-style apartment is adorned with 24-carat gold and marble. There are crystal chandeliers and murals painted on the ceilings.

In 2013, Melania told ABC News that their son, then 7, was “not a sweatpants child,” boasting that she rubs the Caviar Complex C6 moisturizer from her skin-care line on his body after his nighttime bath.

When not taking care of Barron, she runs her own jewelry business, which she sells on QVC in her still-thick Eastern European accent.

“I started my business when he started school,” she said. “When he is in school, I do my meetings, my sketches and everything else.”

Her third job is being the better half — the more low-key, quiet half — of Donald Trump.

“You know, to marry a man like Donald, you need to know who you are,” Melania told Larry King during a 2005 interview with both Trumps.

Giving her hubby a steely gaze, she added, “You need to know who you are, and you need to be very strong and smart.

“And . . . he needs to know that he could rely on me sometimes.”

Friends and acquaintances say she’d make a great first lady.

“She has a quiet, gracious manner, and she is remarkable because of her beauty,” says David Patrick Columbia, founder of New York Social Diary, a society-news site.

“She’s European, and I have a sense that she’s like a lot of European women. They have that talent for standing back and letting him be up front.”

Melania, who reportedly speaks four languages, rarely wades into politics herself.

Although in 2011, when Trump was pushing for President Obama to release his birth certificate, Melania — who became an American citizen in 2006 — backed up her husband in an interview with Joy Behar.

“In one way, it would be very easy if President Obama just show it,” she said. “It’s not only Donald who wants to see it; it’s American people who voted for him and who didn’t vote for him.”

At the moment, behind the scenes is where Melania seems to be making an impact. In June, Trump confessed that she told him to stop attacking fellow Republican candidate Jeb Bush, likely because they are friendly on the charity circuit.

“I was actually told that by my wife,” he told CNN’s Jake Tapper.

And if The Donald does make it all the way to the White House, don’t worry about another Monica Lewinsky scandal — or at least one that the first lady will stick around for.

Asked by Larry King if she ever worried about Trump cheating, Melania said: “No. I don’t worry about that at all. I know who I am, and if a man doesn’t want to be with me or I don’t want to be with a man, [goodbye].”

In other words, the former shy girl from Slovenia would tell the president of the United States, “You’re fired!”