Jimmy and Rosalynn Carter were the longest-married presidential couple. They married in 1946, when he was 21 and she was 18.

Rosalynn Carter, the former first lady of the United States and Jimmy Carter’s closest adviser during his only term as president, has died at the age of 96.

The Carter Center said Sunday she died “peacefully and with her family by her side” at her rural home in Plains, Georgia, after living with dementia and suffering from poor health for many months.

“Rosalynn has been my equal partner in everything I have ever accomplished,” 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 someone loved and supported me.”

Jimmy and Rosalynn Carter were the longest-married presidential couple. They married in 1946, when he was 21 and she was 18.

After his term ended in 1981, he also enjoyed more years in the White House than any president before him, and she played crucial roles during those years, including as part of the nonprofit Carter Center and the charity Habitat for Humanity.

Before coming to Washington in 1977, she was considered modest and quiet, but grew into an eloquent speaker, activist and campaigner.

Her abiding passion, which extended well beyond her years in the White House, was for people with mental illness, not out of a personal connection but out of a strong sense that advocacy was needed.

Before Jimmy Carter was elected president in 1976, Roslynn was largely unknown outside of Georgia, where her husband was a peanut farmer and then governor.

He served a four-year term as a Democrat and lost re-election in 1980 to Ronald Reagan, a Republican former California governor and Hollywood actor.

In Washington, D.C., the Carters were a team, and the president called her “an extension of myself” and “my closest advisor.” She was often invited 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 I do,” he said.

The first lady was also sent on important official missions to Latin America and was part of the unsuccessful campaign to ratify the Equal Rights Amendment to the U.S. Constitution to ensure women’s equal treatment under the law.

The Iran hostage crisis – in which American diplomats and others were held captive in Tehran after the Islamic revolution – came as Carter was seeking re-election. The crisis contributed to the failure of his presidency as he abstained from campaigning while attempting to resolve the stalemate.

During this time, Rosalynn Carter attempted to support her husband by speaking in 112 cities in 34 states during a 44-day tour.

Her speeches and forays into crowds were credited with helping Carter defeat Democratic challenger Ted Kennedy in the 1980 primary, although he subsequently lost overwhelmingly to Reagan in the general election.

Her interest in mental health issues arose in the early 1970s when she began to realize the magnitude of the problem in Georgia and people’s reluctance to talk about it.

As First Lady of Georgia, she served on a gubernatorial commission to improve mental health services.