New York Times bestselling author Jerry Oppenheimer, who recently published RFK Jr.: Robert F. Kennedy, Jr. and the Dark Side of the Dream, investigated the Clintons in his book State of a Union: Inside the Complex Marriage of Bill and Hillary Clinton.

She was a pretty 21-year-old political science major, and he was a 27-year-old law school graduate embarking on his first political campaign. They instantly fell for each other. But when the other woman in his life learned of their affair, things turned frighteningly ugly.

It may sound like the story line for a juicy TV miniseries, but it was real-life. The characters in the drama were a young Bill Clinton, his then longtime girlfriend, Hillary Rodham, and a college girl by the name of Marla Crider.

Before her death last year, Crider sat down for an explosive interview that revealed the perverse lengths Hillary would go to ensure that her political ambitions for herself and her husband-to-be would be realized.

The Clinton-Rodham-Crider triangle is important in the Clinton biography because it underscores and documents the first time Bill cheated on Hillary -- even before they were married, but seriously involved.

Driven and ambitious, Hillary did every catty thing to break up the relationship and keep Bill for herself, based not on love, as Marla Crider perceived it back then in 1974 Arkansas, but rather on a 'secret pact' that Bill and Hillary had forged for their future – that they'd work together as a disciplined team so that one day he, and hopefully later she, would be elected president.

Love triangle: Marla Crider was an Arkansas beauty queen who captured the heart of Bill Clinton even though he and Hillary was about to become engaged. The Clinton-Rodham-Crider triangle is important in the Clinton biography because it documents the first time Bill cheated on Hillary -- even before they were married

Charasmatic: Bill's charm won over many women as he launched his political career in 1974 after he graduated from the University of Arkansas Law School, running for the state's Third Congressional District seat against a popular Republican who had been in office seemingly forever

Long before the so-called Clinton 'bimbo eruptions,' long before the Monica Lewinsky sex scandal, Bill Clinton had, behind Hillary's back, a serious affair with pretty Arkansas brunette Marla, who was working in his first political campaign.

GOP front runner Donald Trump has zeroed in on the Clintons' past marital scandals, asserting the former president's womanizing - in particular his affair with the one-time White House intern Lewinsky - is 'fair game' as the campaign enters its key months.

Declared Trump, 'You look at whether it's Monica Lewinsky or Paula Jones or many of them. That certainly will be fair game. Certainly if they play the woman's card with respect to me.'

When Trump learned that Bill is campaigning for Hillary, he exploded, tweeting that the former president and future possible 'First Husband' has 'demonstrated a penchant for sexism' – this after Hillary called Trump 'sexist' in response to his claim that she was 'schlonged' by Barrack Obama in her failed 2008 presidential race.

In a radio interview with broadcaster Aaron Klein, Monica Lewinsky confidant Linda Tripp revealed that Bill had flings with thousands of women, including another White House Staffer and, of course, Gennifer Flowers.

Bill has also had sexual assault charges leveled at him by Kathleen Willey and Paul Jones, who won an $850,000 lawsuit against him for sexual harassment.

Now the Clintons fear - and justifiably so - that Bill's relationship with Marla, along with the other women in his life, will be resurrected by Trump's campaign and used as fodder against Hillary in the race for the White House.

Slender, olive-skinned, green-eyed Marla Crider's relationship with Bill flew under the radar until it was revealed in my New York Times bestselling book, 'State of a Union: Inside the Complex Marriage of Bill and Hillary Clinton,' published following Clinton's impeachment by the House of Representatives, and acquittal by the Senate.

Secrets: Hillary has stood by her cheating man since before they were married. Monica Lewinsky's confiante Linda Tripp has revealed that Bill had 'thousands' of women - and Hillary knew about them

A wing and a prayer: Hillary and Bill were a match made in political heaven and they had a 'secret pact' for their future that one day he, and later she, would be elected president

Rah rah: Crider was a cheerleader at Greenland High School in Arkansas. She went on to attend the University of Arkansas which is where she met Bill

A native of Bill Clinton's home state, Arkansas, Marla was a sharply intelligent college girl majoring in political science at the University of Arkansas when, in 1974, she joined charismatic, idealistic and seemingly unattached Bill Clinton's first political campaign.

The recent law school graduate was running for the state's Third Congressional District seat against a popular Republican who had been in office seemingly forever.

Everywhere Bill campaigned, there were women, groupies, drawn to him like ants to a picnic. Bill Clinton was a virtual babe magnet, and it was clear to his campaign manager, Paul Fray, and campaign office workers that he took advantage of the situation.

'They'd [women] write down their names, their phone numbers, their addresses on a napkin, whatever was handy, and hand it to him,' one campaign source recalled.

And while he was getting all that action, his girlfriend Hillary was hundreds of miles away in Washington, D.C. working for the Watergate Committee.

Marla Crider told Jerry Oppenheimer she considered Bill hers

But of all the Arkansas cuties who let Bill know they were available, he chose Marla Crider, with her intriguing looks and long dark hair.

The first time they met, she later recalled in interviews for my book, it was practically love at first sight. She was so impressed with him that, 'I told my mother that night, "Bill Clinton will someday be president." I just instinctively felt like the sky was the limit for him.'

A number of books about the Clintons before mine had mentioned a 'college girl' who was romantically involved with Clinton during his first campaign, and had to be hidden whenever Hillary showed up.

But no one had ever found her.

As she later noted to my researcher who tracked her down: 'In 25 years I've never talked about this.'

But she finally decided to open up.

She said she knew that Hillary Rodham, then working in Washington for the Watergate committee, was Bill's girlfriend, and had asked him 'point-blank if he were engaged to her yet.' Bill said no, noting, 'If she chose to come back, I would probably pursue the relationship. But we're not engaged.'

Marla said she believed him. She considered him hers.

But when she later asked him about Hillary she got quite a different response.

'You and Hillary, what's the deal? Are you in love with her?'

'Yeah, I am,' he responded. 'But I don't know if this is right. …I don't know if she can fit in here.'

'What is it about her?' Marla asked.

Bill's response surprised her.

'She gets me started, kicks my butt, and makes me do the things I've got to do.'

To Marla it sounded more like Bill was discussing a tough Marine Corps drill instructor rather than a woman with whom he claimed he was in love with.

'Are you going to build a marriage off of this?' Marla asked dubiously.

Hillary had become aware of the romance – presumably from intelligence gathered from people she knew in the campaign.

And she saw Marla as genuine threat to her future with Bill

Stumped: Bill is campaigning for his wife in the hope that he'll fulfill his promise to help her win the presidency as she had helped him. But Hillary is on the ropes as Bernie Sanders closes in

Vows: They looked so in love on their wedding day. But it was more business than pleasure. Bill told Marla: 'She gets me started, kicks my butt, and makes me do the things I've got to do'

Her claws were out and a number of a confrontations followed. Hillary made surprise visits to Bill's campaign headquarters, in one instance causing Bill to cancel an overnight trip with Marla.

That night at headquarters Marla was drinking a glass of wine.

Hillary, she recalled, 'Strolls up to me, takes the glass of wine out of my hand, says, 'Hmmm, wine' – takes a drink of it, gives it back to me and says, staring me down, "I'll have to get some of that."

'It was her way of letting me know – like a cat – marking its territory. Everybody in there just turned and looked, like what the hell was that all about? And Hillary just had this kind of half smirk on her face and she strolled off.'

It was her way of letting me know – like a cat – marking its territory.

Once when Marla and Bill were discussing his campaign schedule with a few others, Hillary, who had made another surprise visit from Washington, cut into the discussion, telling Marla that she and Bill were going on an overnight trip, 'And we can't seem to find any of his socks. I knew you would know where they are.'

Bill, embarrassed, went ballistic.'Would you two take this outside,' he yelled.

Marla recalled, 'He was red, that pulsating red.'

Marla found the overnight bag, found a pair of Bill's socks inside and briskly gave them to Hillary.

Responded Hillary with poisonous sweetness, 'Thanks – somehow I knew you'd know that.'

Bill later apologized to Marla for Hillary's behavior.

'Basically, he said, "This is my fault. I have allowed this to happen." It was apparent, he said, this was going to have to be dealt with.'

But Marla's relationship ignited more serious problems.

An anonymous telephone call to her mother – whether it was from Hillary, or possibly an agent of hers, or someone else – alerted her to the affair, and she was upset, fearful that her daughter, in her first serious relationship, would be hurt by Clinton.

'Doesn't he have another woman…You are being used,' Mrs. Crider told her daughter during a tearful confrontation.

Beyond other anonymous phone calls and hang-ups, Marla began to suspect she was being followed whenever she left Clinton's campaign headquarters.

Marla had become very aware of Hillary's wrath, and even a bit fearful.

Hillary and Bill Clinton attended Yale in the early 1970s. She was one of a handful of young researchers and interns who helped determine whether a school in Alabama discriminated against blacks

At one point, Bill sent her a note:

'I know there are things about us I should really be sorry about, but I'm not. I care about you. I am grateful that you have been here with me during this time.'

After Hillary found out about Marla, she tried to make Bill jealous.

Campaign manager Paul Fray recalled overhearing a telephone conversation between Hillary and Bill.

'She told him she'd gone out with some guy in Washington and slept with him. Billy broke down and told her, "Well, damn you, why are you doing me this way?" He was really torn up about it.'

She [Hillary] didn't know how to deal with Southerners. She would get furious if there was any frivolity. Furious – it was like have you forgotten what our goal is.

They had a curious unbroken – and unspoken --bond.

Marla's relationship with Bill caused problems within her own family.

At one point her uncle, a county judge, confronted her at restaurant. 'He tore into her a** with both feet,' Fray recalled. 'He said, "You are going to stop this relationship…Everybody's talking about what kind of a sorry, no-account niece I've got, to be with this son of a bitch."'

Marla refused to listen. She was head over heels in love.

Hillary had been hopping in and out of Arkansas to be with Bill. But now, concerned about his relationship with Marla, she returned full time.

'There were few people she tolerated in that office,' Marla had never forgotten. 'She was very dictatorial. She literally came in and just turned everything around, changed everything.

'She didn't know how to deal with Southerners…She would get furious if there was any frivolity. Furious – it was like have you forgotten what our goal is.'

Another campaign aide remembered Hillary, 'kicking a**, and taking names'.

Bill had given Marla her own set of keys to his house and had asked her to pick up some papers he needed on one occasion when Hillary had apparently returned to Washington.

On Bill's desk in his room Marla spotted a letter signed by Hillary, unfolded and open to be read by anyone. Years later, remembering that moment, Marla believed that Hillary had 'carefully placed' it there, in what she believed was a Hillary 'orchestrated touch'

On Bill's desk in his room Marla spotted a letter signed by Hillary, unfolded and open to be read by anyone.

Years later, remembering that moment, Marla believed that Hillary had 'carefully placed' it there, in what she believed was a Hillary 'orchestrated touch.'

Marla recalled much of what Hillary had written in her Dear Bill missive, and most, if not all of it, was a shock to her.

Wrote Hillary: 'I still do not understand why you do the things you do to hurt me. You left me in tears and not knowing what our relationship was all about.

'I know all your little girls are around there…if that's what it is, you will outgrow this. They will not be with you when you need them.

'They are not the ones who can help you achieve your goals. If this is about your feelings for [Marla] this, too, shall pass. Let me remind you it always does.'

Reading the words, Marla believed that Hillary was saying that what Bill and Marla had together wasn't real, that he should listen to his head, not his heart.

Hillary continued, 'Remember what we talked about. Remember the goals we set for ourselves. You keep trying to stray away from the plans we've put together.

'Take some time, think about it, and call me when you're ready. Hillary.'

In November 2014, as 68-year-old Hillary Clinton was launching her second presidential bid, Marla Crider died at age 60 after a heroic 18-month battle against a form of breast cancer

Marla said she found the letter chilling, that Hillary's words seemed to have nothing to do with love, but rather about an ambitious secret pact for their future.

Realizing her relationship with Bill Clinton would go nowhere with Hillary in the picture – and the strange pact that they shared -- she decided to end it with him.

'It was very adult, very above board. He said, 'You need to take care of yourself. You just don't know how much you mean to me.'

Looking back years later to her relationship with the future president, she said, 'It was a period of time when he was not married, even though they had an understanding, she made that perfectly clear.

'I'm not so sure those weren't some of the first real feelings he'd had, but then we got the game plan enacted and we'd seen what's it been since then.

'So – how lonely has he been? How lonely has she been? Do they love each other? I think maybe the only way they know how. But I think for a short period of time, early on, maybe, he may have been willing to love in a different way.'

In November 2014, as 68-year-old Hillary Clinton was launching her second presidential bid, Marla Crider died at age 60 after a heroic 18-month battle against a form of breast cancer called invasive ductal carcinoma (stage 3).

Hundreds of people attended a memorial service for her in Fayetteville, Arkansas.