Inside Rosalynn Carter and Betty Ford's Unstoppable 40-Year Friendship: 'A Formidable Duo'

It was the mid-1970s when Rosalynn Carter, then the first lady of Georgia, first met Betty Ford, then the second lady of the United States.

Ford was traveling through the Peach State on an official trip that saw her bringing art to rural communities across the country, and Rosalynn and her husband, Democratic Georgia Gov. Jimmy Carter, invited her to stay at the governor's mansion.

"She was the most distinguished guest we had ever had, but when she arrived, she was so warm and friendly that she immediately put me at ease, and we had a good time together," Rosalynn once recalled of Betty, who would soon be promoted to first lady when her husband, Republican Vice President Gerald Ford, took over as president of the United States upon Richard Nixon's resignation.

A couple of years later, the Carters and Fords would be pitted against one another as Jimmy faced off with Gerald in the 1976 presidential election, ultimately unseating him and sending the Fords packing from the White House.

That's when the two independently minded political wives met again.

In a eulogy at Betty's 2011 funeral, Rosalynn acknowledged that it could have been "a very awkward moment" running into the Fords under those circumstances, were it not for Betty's warmhearted nature. "I know from personal experience that it was a difficult time for her, yet she was just Betty, as gracious as always," Rosalynn said at the time.

When Rosalynn became first lady immediately after Betty, she said she turned to her new friend for advice and guidance: "As I assumed the responsibilities of first lady, I had an excellent role model and a tough act to follow."

Far more than a president's wife, Rosalynn is remembered as a spirited first lady — one whose legacy long outlasted her time in the White House through numerous humanitarian projects that continue to this day.

But before Rosalynn, there was Betty, who set a precedent as a politically active presidential spouse and made her own mark on the national conversation. As first lady, Betty worked to break down stigmas surrounding a number of issues, often challenging the values of the Republican Party and going against the wishes of her husband's advisers. She raised awareness about the risk of breast cancer by walking the American people through her treatment, she publicly declared her support for a woman's right to abortion, she openly discussed the benefits of psychiatry, and she expressed understanding for people who engaged in marijuana use and premarital sex.

Rosalynn, on the heels of Betty's groundbreaking 895 days as first lady, stepped into her role at the White House ready to continue sparking conversation, choosing mental health and women's rights as two of her primary causes and pushing the boundaries of what a first lady could do by getting heavily involved in her husband's administration.

Kathryn Cade — who served as Rosalynn's director of projects while she was in the White House — explains to PEOPLE the power of Rosalynn and Betty's nearly 40-year friendship.

"The two women initially bonded over their devotion to their families and support for their political husbands, then later joined forces to become a powerful bipartisanship team to promote women’s rights and passage of the Equal Rights Amendment," says Cade, who now serves as vice chair of the Carter Center’s Board of Trustees.

The two also found common ground in their public health interests — Betty's in substance abuse and Rosalynn's in mental health. If Betty asked for anything, Rosalynn was known to respond affirmatively — and vice versa.

While Betty advocated for Rosalynn's focus on mental health, Rosalynn would champion the passion project close to Betty's heart: the Betty Ford Center, a nonprofit, residential treatment center for those with substance dependence.

Over time, the two often partnered up, traveling to Washington to lobby for their causes. In March 1994, the duo testified before Congress in support of including mental health and substance abuse treatment benefits in the national health care reform plan.

"After the White House, through The Carter Center and the Betty Ford Center, they united to speak on behalf of the millions of Americans living with mental health and substance use conditions," Cade tells PEOPLE.

"Mrs. Carter often talked about how much fun the two of them had educating members of Congress about mental health parity legislation," she adds. "They would go together to publicly present their case, then each would visit legislators on her respective side of the aisle to solidify support. They were a formidable duo.”

It was Rosalynn who nominated Betty for induction into the National Women’s Hall of Fame, and who, in January 2003, attended the 20th anniversary celebration of the Betty Ford Center alongside three other first ladies: Nancy Reagan, Barbara Bush, and Hillary Clinton.

And in July 2011, it was Rosalynn who delivered the eulogy at Betty's funeral, held in Saint Margaret Episcopal Church in Palm Desert, California.

"I never imagined when we first met 40 years ago that we would develop such a close personal friendship," Rosalynn said.

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Elsewhere in her eulogy, Rosalynn recounted how Betty wrote her a note after the two participated in a 1984 panel of first ladies. In it, Rosalynn said, Betty "expressed her admiration for women who had the courage of their convictions and did what others were too timid to attempt."

Betty, Rosalynn added, was "someone who was willing to do things a bit differently than they had been done before, someone who had the courage and grace to fight fear, stigma, and prejudice wherever she encountered it."

And while the women's husbands were known to have a close relationship as well, Rosalynn suggested hers and Betty's may have rivaled it.

"Historians have said that our husbands, Jimmy and Jerry, developed a closer relationship than any other presidents after leaving the White House," she said. "I think Betty and I had a similar relationship."