It’s Ivana Trump’s fault that Donald didn’t run for president sooner.

“Probably five years before our divorce, Reagan or somebody brought him a letter and said, ‘You should run for president,’ ” Donald’s first wife tells The Post at her opulent seven-floor Upper East Side town house, which she purchased for $2.8 million in 1998.

“So he was thinking about it. But then . . . there was the divorce, there was the scandal, and American women loved me and hated him,” she says, referencing Donald’s much-publicized infidelity with Marla Maples that led to the power couple’s 1991 split. “So there was no way that he would go into [politics] at that point,” Ivana says. “But he was always tooling around with the idea.”

No tooling now — now, Donald’s the GOP front-runner.

And Ivana, a self-proclaimed conservative who has mended fences with her ex, is along for the ride — acting as Donald’s cheerleader and, at times, adviser.

“I suggest a few things,” says Ivana, lounging next to her framed 1996 “Got Milk?” ad in the Louis XVI-inspired living room — the fireplace blazing on a 70-degree spring day.

The 67-year-old former model is sporting a mini Roberto Cavalli dress with sheer lace panels, an Ivana Trump necklace from her defunct QVC jewelry line (“Now I just prefer to take it easy,” she says of getting out of the accessories business) and her signature blond bouffant.

“We speak before and after the appearances and he asks me what I thought,” says Ivana, who tells him to “be more calm” and adds that she gave Donald the motto: “You think it, I say it.”

“But Donald cannot be calm,” she admits. “He’s very outspoken. He just says it as it is.” That’s exactly why she thinks The Donald — as she famously referred to him in a 1989 Spy magazine interview — would make a great president.

“He’s no politician. He’s a businessman. He knows how to talk. He can give an hour speech without notes . . . He’s blunt.” She adds that Donald would surround himself with “fantastic advisers, like Carl Icahn. Really brilliant minds. And he’d make a decision! Obama cannot make a decision if his life depends on it. It’s ridiculous.”

While Ivana says she’s not political — “I was born in a Communist country [Czechoslovakia] and I don’t like politics” — she frets that America has lost its prestige. “We have to get it back.”

The first step? Adopting Donald’s immigration policies. “And I’m an immigrant,” Ivana says.

“I have nothing against Mexicans, but if they [come] here — like this 19-year-old, she’s pregnant, she crossed over a wall that’s this high” — Ivana lowers her hand to 4 inches above her wall-to-wall carpeting. “She gives the birth in American hospital, which is for free. The child becomes American automatically. She brings the whole family, she doesn’t pay the taxes, she doesn’t have a job, she gets the housing, she gets the food stamps. Who’s paying? You and me.

“As long as you come here legally and get a proper job . . . we need immigrants. Who’s going to vacuum our living rooms and clean up after us? Americans don’t like to do that.”

Well, Trumps certainly don’t.

Ivana wasn’t always a Trump, though. Born Ivana Zelnícková, she grew up in what was then Czechoslovakia, and began skiing competitively at age 6. She says she was an alternate for the 1972 Czechoslovak Olympic ski team (there are disputes as to the validity of this claim). She was married from 1971 to 1973 to an Austrian skier, Alfred Winklmayr, and lived in Montreal, where she modeled and worked as a ski instructor.

In 1976, Ivana was in NYC for a fashion show and went with friends to Maxwell’s Plum, an Upper East Side pickup spot for real-life Mr. Bigs and the women who loved them. While waiting for a table, she felt a tap on her shoulder.

“[There’s] this tall blond guy with blue eyes. He said, ‘I’m Donald Trump and I see you’re looking for a table. I can help you.’ I look at my friends and said, ‘The good news is, we’re going to get a table real fast. The bad news is, this guy is going to be sitting with us.’ ”

After the meal, Donald paid the bill on the sly and disappeared.

“I said, ‘There’s something strange because I’ve never met a man who didn’t want anything from a woman and paid for it,’ ” Ivana says with a laugh.

When she walked outside, there was Donald, in the driver’s seat of his own limousine. “He drove us home and then we started to date,” she says.

‘As long as you come here legally … We need immigrants. Who’s going to vacuum our living rooms and clean up after us?’ - Ivana Trump

Donald took Ivana to Aspen, Colo., not knowing her Olympian status. She feigned altitude sickness while Donald, a ski novice, took a lesson.

“Sure enough, 10 minutes later I was on the mountains and looking at him doing full turns, and it was not fun,” says Ivana.

“The second day, he was getting good. So he said, ‘Ivana, let’s go ski.’ I asked the instructor to put the ski boots on me like a beginner. Donald was like, ‘OK, darling, you can do it!’ I took off and he got so angry. He said, ‘I will never [ski] again for anybody! Even Ivana!’ So I play for his ego.”

After less than a year, the two married, in 1977, and went on to have three children together: Donald Jr., now 38, Ivanka, 34, and Eric, 32. Donald hired his wife to work within his Trump Organization. She was crowned vice president of interior design, eventually becoming president of the Trump’s Castle casino resort, in Atlantic City, and later of the Plaza hotel.

“I’d get up at 6 and read newspapers, and then I went to 63rd Street to the helicopter pad and flew to Atlantic City. I could come back 5 or 6 . . . Then we’d watch ‘Dallas’ and ‘Dynasty.’ ”

Ivana says Donald’s faith in her business acumen is proof of his respect for women. “When people ask, I say that behind every successful woman is a man in shock. And it’s true, so I think Donald knew that I could achieve.

“He gave me the chance. I came [to America] and I was a poor person. I had a sense of style so he put me in charge of interior design of the Grand Hyatt Hotel. I was seven months pregnant and going up the steps, making sure everything was on the schedule.”

“Speaking of America, Donald’s on the phone,” interrupts Dorothy, Ivana’s longtime assistant and her kids’ former nanny. Ivana excuses herself.

“I don’t think he’s feminist,” Ivana says upon her return when asked of Donald’s stance. “He loves women. But not a feminist.” (Ivana’s reps called The Post two days after the interview to clarify that Donald was a feminist. Then they called to say he wasn’t. An hour later, they said he was.) It was that love of women that led to the couple’s divorce. Ivana discovered that her husband was cheating on her with former beauty queen Marla Maples. As Ivana told Barbara Walters in a 1991 “20/20” interview, Maples stopped her at a restaurant in Aspen and told her, “I’m Marla and I love your husband. Do you?”

Ivana filed for divorce, claiming in her deposition that Donald raped her after he used her plastic surgeon for a scalp-reduction surgery to remove a bald spot. “Your f – – king doctor has ruined me!” Trump cried, according to the 1993 book “Lost Tycoon: The Many Lives of Donald J. Trump,” before forcing himself on her sexually.

Once the book was out, Ivana softened her remarks in a statement: “As a woman, I felt violated, as the love and tenderness, which he normally exhibited toward me, was absent. I referred to this as a ‘rape,’ but I do not want my words to be interpreted in a literal or criminal sense.”

Now, she tells The Post, “It was all the lawyers. The negotiations and stuff like that. I was never abused.”

After battling it out in court — and in the tabloids — Ivana walked away with a $14 million cash settlement, the family’s 45-room Greenwich, Conn., mansion, an apartment at Trump Plaza, and use of Donald’s Palm Beach mansion, Mar-a-Lago, every March.

“Donald took the divorce as a businessman. And he had to negotiate and he had to win,” says Ivana. “Once the financial part was settled, we’re friendly.”

She had custody of the children, but he saw them often.

“Once [the children] were the age of 21 and out of the university, I said, ‘Donald, this is the final product, now you deal with it.’ ” She still oversaw the kids’ monthly allowance, though. “Sometimes they’d say, ‘But I have this Arab friend who drives a Ferrari! And is in Armani suits and gets $20,000! And I get $1,500 a month!’ And I said, ‘You know what, the money doesn’t fall from the trees,’ ” says Ivana, who foresees her children running the family business if Donald is in the White House.

As for Donald’s current wife, Ivana thinks Melania will “be OK” as first lady. “She’s going to adapt just fine,” says Ivana, who is friendly with Melania. “She never did anything wrong to me. I wish them all the best.”

She can’t say the same for Maples.

“[Maples] asked to apologize to me in the Daily Mail in London. They asked if I accepted the apology and I said no. Why should I? She broke my marriage!”

Not that Ivana’s too concerned. She’s plenty busy juggling her properties in Saint-Tropez, London, Miami and New York, as well as her three boyfriends.

“If you are a married woman, you usually follow what the man wants to do. I can do whatever what I want,” she says. “I’m not getting married again. But I like companions. The most important for me is honesty, good humor — not necessarily a millionaire. I don’t need [money]. I prefer to be a baby sitter than a nursemaid. I don’t want to worry about bad knees and bad back.”

Plus there’s her bundle of grandchildren — now up to eight, with last week’s addition of Theodore, son of Ivanka and her husband, Jared Kushner. The grandchildren call her “Ivana-ma or Glam-ma,” says Ivana, who plans to attend Theodore’s bris today but says she doesn’t know whether Donald will make it.

While her ex’s campaign has thrust the family into the solar core of the public spotlight, she says the kids and she — and certainly Donald — are used to the heat. Plus, there are surprise benefits, like Donald’s recent weight loss, aided by his germophobia.

“He loves to eat. I told him, ‘Donald, you lost weight!’ because I can tell if he’s 225 or 215,” says Ivana. “And he said, ‘I don’t have time to eat! I shake so many hands!’ ”

Speaking of hands — and other body parts — Ivana says Donald does just fine in that department.

“If there was a problem there, Donald would not have five kids.”