Former first lady Rosalynn Carter, the closest adviser to Jimmy Carter during his one term as US president, has died aged 96.

The Carter Centre said she died on Sunday after living with dementia and suffering many months of declining health.

A spokesperson said she “died peacefully, with family by her side” at 2.10pm local time at her rural Georgia home.

“Rosalynn was my equal partner in everything I ever accomplished,” Mr Carter said in a statement. “She gave me wise guidance and encouragement when I needed it. As long as Rosalynn was in the world, I always knew somebody loved and supported me.”

President Joe Biden called the Carters “an incredible family because they brought so much much grace to the office”. He also spoke of the couple’s “great integrity”.

“Imagine they were together for (77) years?” Mr Biden added. “God bless them.”

The Carters were married for more than 77 years, forging what they both described as a “full partnership”.

Unlike many previous first ladies, Rosalynn sat in on Cabinet meetings, spoke out on controversial issues and represented her husband on foreign trips. Aides to Mr Carter sometimes referred to her — privately — as “co-president”.

“Rosalynn is my best friend… the perfect extension of me, probably the most influential person in my life,” Jimmy Carter told aides during their White House years, which spanned from 1977-1981.

Fiercely loyal and compassionate as well as politically astute, Mrs Carter prided herself on being an activist first lady, and no one doubted her behind-the-scenes influence.

When her role in a highly publicised Cabinet shake-up became known, she was forced to declare publicly: “I am not running the government.”

Many presidential aides insisted that her political instincts were better than her husband’s — they often enlisted her support for a project before they discussed it with the president.

Her iron will, contrasted with her outwardly shy demeanour and a soft southern accent, inspired Washington reporters to call her “the Steel Magnolia”.

Both Carters said in their later years that Rosalynn had always been the more political of the two.

After Jimmy Carter’s landslide defeat in 1980, it was she, not the former president, who contemplated an implausible comeback, and years later she confessed to missing their life in Washington.

Mr Carter trusted her so much that in 1977, only months into his term, he sent her on a mission to Latin America to tell dictators he meant what he said about denying military aid and other support to violators of human rights.

She also had strong feelings about the style of the Carter White House. The Carters did not serve hard liquor at public functions, though Rosalynn did permit US wine. There were fewer evenings of ballroom dancing and more square dancing and picnics.

Throughout her husband’s political career, she chose mental health and problems of the elderly as her signature policy emphasis. When the news media did not cover those efforts as much as she believed was warranted, she criticised reporters for writing only about “sexy subjects”.

As honorary chairwoman of the President’s Commission on Mental Health, she once testified before a Senate sub-committee, becoming the first first lady since Eleanor Roosevelt to address a congressional panel.

She was back in Washington in 2007 to push Congress for improved mental health coverage, saying: “We’ve been working on this for so long, it finally seems to be in reach.”

She said she developed her interest in mental health during her husband’s campaigns for Georgia governor.

“I used to come home and say to Jimmy, ‘Why are people telling me their problems?’ And he said, ‘Because you may be the only person they’ll ever see who may be close to someone who can help them,’” she explained.

After Ronald Reagan won the 1980 election, Mrs Carter seemed more visibly devastated than her husband. She initially had little interest in returning to the small town of Plains, Georgia, where they both were born, married and spent most of their lives.

“I was hesitant, not at all sure that I could be happy here after the dazzle of the White House and the years of stimulating political battles,” she wrote in her 1984 autobiography, First Lady From Plains.

But “we slowly rediscovered the satisfaction of a life we had left long before,” she said.

After leaving Washington, Jimmy and Rosalynn co-founded The Carter Centre in Atlanta to continue their work.

Frequently, the Carters left home on humanitarian missions, building houses with Habitat for Humanity and promoting public health and democracy across the developing world.

“I get tired,” she said of her travels. “But something so wonderful always happens. To go to a village where they have Guinea worm and go back a year or two later and there’s no Guinea worm, I mean the people dance and sing — it’s so wonderful.”

In 2015, Mr Carter’s doctors discovered four small tumours on his brain. The Carters feared he had weeks to live. He was treated with a drug to boost his immune system, and later announced that doctors found no remaining signs of cancer. But when they first received the news, she said she did not know what she was going to do.

“I depend on him when I have questions, when I’m writing speeches, anything, I consult with him,” she said.

She helped Carter recover several years later when he had hip replacement surgery aged 94 and had to learn to walk again. And she was with him earlier this year when he decided after a series of hospital stays that he would forgo further medical interventions and begin end-of-life care.

Jimmy Carter is the longest-lived US president. Rosalynn Carter was the second longest-lived of the nation’s first ladies, trailing only Bess Truman, who died aged 97.