Former US First Lady Rosalynn Carter, Jimmy Carter’s closest adviser during his single term as President of the United States and in the four decades that followed, has died at the age of 96.

The Carter Center announced she died Sunday after suffering from dementia for many months and her health deteriorating.

Married for more than 77 years, the Carters forged what they both referred to as “… full partnership. Unlike many other first ladies, Rosalynn Carter attended Cabinet meetings, spoke on controversial issues, and represented her husband on his trips abroad. President Carter’s staff 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 lifeJimmy Carter told his staff during their years in the White House, from 1977 to 1981.

Rosalynn Carter was fiercely loyal, compassionate and politically astute. She was proud to be an activist first lady, and no one doubted her influence behind the scenes. When her role in a high-profile restructuring was revealed, she was forced to publicly declare: I don’t run the government.

Many of the president’s aides insisted that their political instincts were better than their husband’s: they often assured his support for a project before discussing it with the president. Her iron will, in contrast to her outwardly shy demeanor and soft Southern accent, had led Washington journalists to give her a nickname The Steel Magnoliabe that Steel Magnolia.

Both Carters later said that Rosalynn was always the more political of the two. After Jimmy Carter’s crushing defeat in 1980, it was she, not the former president, who pondered an unlikely comeback. Years later, she confessed that she missed her life in Washington.

Jimmy Carter is the longest-living American president. Rosalynn Carter is the second oldest first lady after Bess Truman, who died at the age of 97.