Former U.S. First Lady Rosalynn Carter, who President Jimmy Carter called "an extension of myself", died on Sunday aged 96.
"Guided by her abiding faith and a commitment to service, Mrs Carter used her platform in profoundly meaningful ways," wrote Michelle Obama, in a post on social media, adding that she was grateful for the good advice and helping hand she received from her predecessor during her own time in the White House.
Rosalynn Carter entered hospice care at home in Plains, Georgia a few days ago, and died with her family by her side, according to a statement released by the Carter Center, a nonprofit organization founded by the couple.
Both Carters were active in the Plains community, including at the Maranatha Baptist Church where Rosalynn served as a deacon and Jimmy as a deacon and long-time Sunday school teacher, even after leaving the White House.
Jimmy Carter, a Democrat, served as president from 1977 to 1981. He and his wife were the longest-married U.S. presidential couple, having wed in 1946 when he was 21 and she was 18.
"The best thing I ever did was marry Rosalynn," Carter told the C-SPAN cable TV channel in 2015. "That's the pinnacle of my life."
Before her husband was elected president in 1976, Rosalynn was largely unknown outside of Georgia, where he had been a peanut farmer-turned-governor. He lost his 1980 re-election bid to Ronald Reagan, a Republican former California governor and Hollywood actor.
In Washington, the Carters were a team, with the president calling her "an extension of myself" and "my closest adviser." She was often invited to sit in as an observer at cabinet meetings and political strategy discussions. In a 1978 interview with magazine editors, Carter said he shared almost everything with his wife except top-secret material.
"I think she understands the consciousness of the American people and their attitudes perhaps better than do I," he said.
President Joe Biden, who served in the Senate during the Carter presidency, and first lady Jill Biden said in a statement that Rosalynn Carter "walked her own path, inspiring a nation and the world along the way."
"She was a champion for equal rights and opportunities for women and girls; an advocate for mental health and wellness for every person; and a supporter of the often unseen and uncompensated caregivers of our children, aging loved ones and people with disabilities," the Bidens said.
Her interest in mental health issues stemmed from the early 1970s when she began to realize, while helping her husband campaign for governor, the depth of the problem in her home state of Georgia and the reluctance of people to talk about it.
In their post-Washington years the Carters were also key figures in the Habitat For Humanity charity, helping build homes for needy families. Their humanitarian efforts were crowned in 2002 when Jimmy Carter was awarded the Nobel Peace Prize.
"I am especially grateful to Rosalynn, who has been a part of everything I've done," a teary-eyed Jimmy Carter said in a speech in Plains after learning he had won the award.
The Carter Center said she also is survived by her four children, 11 grandchildren and 14 great-grandchildren.
(with additional reporting by Reuters)