Jackie Kennedy's filthy rich best friend helped former Democratic presidential candidate John Edwards keep his affair and love child under wraps, but absolutely despised another Democratic presidential hopefull: Hillary Clinton.

Listerine heir Rachel “Bunny” Mellon started her decades-long friendship with Kennedy when Jackie was just a young senator's wife.

“Jackie was looking for someone who could guide her and give her advice,” Meryl Gordon told Fox News. “These were both women who didn’t have a million women friends. They were really looking for someone they can trust. They were both discreet, so they were able to talk about their difficult marriages.”

Gordon, a professor at NYU who interviewed Mellon in 2011, recently published a book about the late heiress, “Bunny Mellon: The Life of an American Style Legend.” Mellon’s family gave Gordon access to private letters exchanged between the two best friends, and also correspondence that shed light on Mellon's relationships with Edwards and Clinton.

Mellon's husband of over 50 years, philanthropist Paul Mellon, died in 1999 at age 91. A year later, her daughter Eliza Lloyd from her first marriage was crossing a street in Manhattan when she was hit by a truck, resulting in a brain injury.

Mellon was alone and depressed — until she discovered someone who reminded her of that young senator of yore, John Kennedy.

"She really admired [former North Carolina Senator] John Edwards,” said Gordon. "John Edwards is a tremendously charming and charismatic young man… At that point in her life, Bunny was excited to be relevant again."

Edwards, who was a candidate for the Democratic presidential nomination in 2008, quickly became friends with Mellon.

"Here she is, home alone with her daughter who's ill," said Gordon. "And here’s this presidential candidate who calls her before attending debates, when he’s on the road. It was exciting for her to become a part of something."

Gordon claimed Mellon gave more than $3 million to his campaign and poverty center. And when Edwards received negative press for spending $400 on a haircut, she offered to cover all of his expenses, despite her lawyer’s disapproval. She was then allegedly approached by Andrew Young, one of Edwards’ closest aids, to provide funding for a “special project.” She blindly provided funds without hesitation.

“At that point, John Edwards’ mistress was pregnant and about to give birth,” said Gordon. “They needed money to support her during this period… [Meanwhile,] Bunny thought this was fun. Her friend wanted money and she kept thinking of how to outwit her advisers… She was worth $760 million. So she came up with this idea that she would send money to her decorator, who would then forward it on to the campaign.”

Mellon, along with the rest of the world, would find out Edwards' secret in 2010 when the then-56-year-old shocked the press and the country with his confession. She reportedly shrugged off the news, claiming all men have affairs. But Edwards’ political career was over.

“Long after Bunny was giving him money … they stayed in touch,” said Gordon. “Bunny called him the last weekend before she died to say goodbye. She kept his picture by her bedside. So for her, this was a relationship that mattered.

"To his credit, he didn’t walk away, even though there was nothing in it for him, other than friendship. [But] her grandson and son were skeptical. They were not enthused. In fact, at Bunny’s funeral, John Edwards came and the family sent word he was not welcomed in the church… They felt he had really taken advantage of an older woman who was gullible to some extent.”

But one person who failed to impress Mellon in her lifetime was former first lady Hillary Clinton.

“During the Clinton-White House years, Bunny’s lawyer had been name head of legal services,” explained Gordon. “There was a whole anniversary celebration at the White House. He brought Bunny as his date.

"Bunny was very used to people being very complimentary about her work in the White House. But when she was introduced to Hillary, whatever was going on with Hillary that day, Bunny didn’t feel she had been appropriately respectful. And Bunny was pretty angry about it… In her letters… she referred to Hillary as ‘the old rag,’ ‘the elf.’ They were pretty negative comments.”

Nevertheless, Mellon made sure to always treasure her friendships, even if they proved to be controversial.

“I think she really enjoyed herself,” said Gordon. “The notion that she still mattered in the world, I think it meant a lot to her.”

And starting with the Kennedy family, Mellon was accustomed to mattering.

Jackie Kennedy met Mellon through a mutual friend, Fred Astaire’s sister and dancing partner Adele Astaire, and was instantly inspired by the woman who was almost 20 years her senior.

“One of the things that was very touching was being able to read some of Jackie’s letters to Bunny,” explained Gordon. “They were so warm, so affectionate. They’re so filled with admiration… You can just really see how much they mattered to each other. And Bunny gave lots of gifts to Jackie.

"She was obsessed with making sure any gift she gave to Jackie was perfect… She once went searching the seashore to find the perfect pair of shells just to put earrings in them… The presentation was always as important as a gift.”

And Kennedy returned the favor with a major honor after her husband John "Jack" Kennedy became president of the United States in 1960.

“The phone rings and it's Jackie,” recounted Gordon from one of Mellon’s handwritten essays. “Jackie says, ‘I can’t talk long, we’re going to church. Jack is going to ask you something today and you have to say yes… He’s going to ask you to design the White House rose garden.’

"And Bunny just talks about wondering, ‘How am I possibly going to do this? Yes, I love gardening, but I don’t have formal training.’ She was very close to her maternal grandfather, who was very patriotic… He told her, ‘You must do it for your country.’ So she went ahead and did it. During the great dig of the garden, they cut a cord that put the whole country on nuclear alert, so there were quite some adventures in that process!”

John was impressed by Mellon’s green thumb.

And when he was assassinated in 1963 at age 46, Mellon made sure to comfort his grieving widow.

“Bunny did the flowers for the capital, where the president’s body laid, for the church, for Arlington cemetery,” said Gordon. “She picked flowers from the rose garden, which the president loved… She also spent time with Jackie during that period when she had to relive every gruesome moment of the assassination. Bunny was willing to listen… They were there for each other when it mattered.”

Mellon would stay by Kennedy’s side until her death in 1994 at age 64.

“Jackie’s sister Lee Radziwill said Bunny was wonderful about being with Jackie in the final couple weeks when Jackie was ill,” said Gordon. “Carly Simon talked to me about being allowed to go to the room and Bunny sitting next to Jackie and holding her hand… They were very close and obviously when someone is nearly 20 years younger than you, you expect to outlive them. It was very hard for Bunny to lose her best friend.”

Mellon passed away at age 103 in 2014.

