Nancy Davis Reagan, 94, the powerful and devoted wife of President Ronald Reagan, principal caregiver during his decadelong twilight with Alzheimer's disease, and the primary architect of his legacy, died Sunday morning at her home in the Bel-Air section of Los Angeles. The cause was congestive heart failure.

She will be buried at the Ronald Reagan Presidential Library in Simi Valley, Calif., next to her husband, who died on June 5, 2004.

Mrs. Reagan is widely viewed, by admirers and detractors alike, as one of the most powerful first ladies in history. Her affection and commitment to President Reagan were resolute. Anyone challenging his authority ultimately answered to his wife, who was equally known for her adoring gaze and her fierce loyalty.

"My life really began when I married my husband," said Mrs. Reagan, who was a Hollywood contract player of middling success before she wed the Screen Actors Guild president in 1952. They made one movie together, Hellcats of the Navy in 1957, the year before she abandoned acting for good.

In her later years, Mrs. Reagan became a tireless campaigner for stem-cell research in developing a cure for Alzheimer's and other diseases, breaking publicly with President George W. Bush and other Republican opponents who favored limited funding.

"I just don't see how we can turn our backs on this," she said at a May 2004 fund-raiser. "We have lost so much time already. I just really can't bear to lose any more."

Although stem-cell researchers applauded her stance, "the most important contributions she and her husband made to Alzheimer's was the very public disclosure of his diagnosis," Jason Karlawish, a doctor and cochair of the University of Pennsylvania's Memory Center, said Sunday. "There's no question that it was done as a couple."

In a public letter issued Nov. 5, 1994, the former president announced to the nation that "I have recently been told that I am one of the millions of Americans who will be afflicted with Alzheimer's disease."

"I feel fine," Reagan wrote, but acknowledged that "as the disease progresses, the family often bears a heavy burden. I only wish there was some way I could spare Nancy from this painful experience."

Karlawish said it was "tremendously courageous of them to issue that letter to the nation because it directly challenged the stigma that was attached to the diagnostic label."

"Not to diminish her advocacy for stem-cell research, but there are a lot of people doing that," he said. "For this public acknowledgment to come from the president of the United States is probably their greatest legacy to Alzheimer's."

An enduring love affair

Nancy Reagan was an actress, a mother, a crusader for several other causes, and an admirer of fine fashion and elite society, but her enduring legacy was as a wife. The Reagans constituted an enduring and public love affair, their affection and devotion for one another absolute.

Ronald Reagan's political success, as a two-term governor of California, from 1967 to 1975, and later as a two-term president, from 1981 to 1989, was bolstered by his wife's unflagging belief in his abilities.

"I think if left to his own devices," their son Ronald Prescott Reagan once said, his father "might have ended up hosting Unsolved Mysteries on TV."

Mrs. Reagan was known for her loyal circle of friends, Hollywood potentates like Merv Griffin and Frank Sinatra - who sang "Nancy (With the Laughing Face)" at state functions - and wealthy philanthropists, including the late Walter and Leonore Annenberg of Philadelphia. The Reagans frequently celebrated New Year's Eve at the former ambassador's Palm Springs estate, Sunnylands.

Mrs. Reagan was also known for leaving a sizable wake of powerful enemies, including former Attorney General Edwin Meese. Former White House chief of staff Don Regan believed he was ousted in 1987 at Mrs. Reagan's behest. He struck back by attacking her the following year in his book, For the Record. "Virtually every major move and decision the Reagans made during my time as White House chief of staff was cleared in advance with a woman in San Francisco who drew up horoscopes," he wrote. Mrs. Reagan's interest in astrology became the grist of political cartoonists and talk-show hosts.

Reagan's popular image

Publicly, President Reagan maintained an avuncular, warm image burnished in the earth tones his wife favored for him, a reputation Mrs. Reagan promoted. In private, the president was an aloof loner who trusted his wife and few others. "I think all his children loved him desperately," Ron Reagan told 60 Minutes in 1999, but the president, he said, needed only Nancy.

It was left to Mrs. Reagan to be his sentry, and often, attack dog. He was the Teflon president. She was the "flypaper first lady," according to their loyal adjutant and public-relations expert, Michael Deaver, and a lightning rod for criticism.

By the end of Reagan's presidency, his wife had run afoul of many of his closest advisers. Thin-skinned and tenacious, Mrs. Reagan wore her emotions and grudges on her Adolfo knit sleeves.

In her 1989 autobiography My Turn, she criticized chief of staff James Baker ("his main interest was Jim Baker") and even her fierce acolyte Deaver, noting he "caught a bad case of Potomac fever."

Kitty Kelley's 1991 scathing biography declared that "for eight years, Nancy Reagan ran this country." The book's attendant publicity and backlash ultimately proved a boon in softening Mrs. Reagan's reputation, with people as disparate as George Will and Joan Rivers rushing to her defense.

Some criticism came from her family, well noted for its dysfunction. The Reagans raised two independent, politically liberal children, Patti and Ron, infrequent White House visitors who spoke openly of their political and emotional differences.

Their daughter wrote several bitter, unsparing Romans à clef and memoirs during her father's presidency that were ironically, given her criticism of Mrs. Reagan's rigid parenting, published under her mother's maiden name. There were also strained relations between Mrs. Reagan and President Reagan's children from his first marriage to actress Jane Wyman, Michael and the late Maureen Reagan.

The family reached a rapprochement during President Reagan's long decline. Mrs. Reagan grew close to her children and stepchildren. She was a tireless, constant nurse and companion to her "Ronnie," eschewing most social requests even after her husband no longer knew her.

In 2000, their lengthy romance was chronicled in the best-selling anthology, I Love You, Ronnie: The Letters of Ronald Reagan to Nancy Reagan.

Always trim and petite - she appeared to be a third the president's size - Mrs. Reagan grew wan and frail, a mere wisp, and suffered from glaucoma. During President Reagan's state funeral, there was an indelible image of her collapsing over his casket before it was lowered into the California soil. Despite her physical fragility, it was clearly the former first lady's event, her last large public moment, as well as his, making sure the casket winged west with the sun, so he could be buried at the Ronald Reagan Presidential Library in Simi Valley at dusk.

Mrs. Reagan traveled a long distance to get to the public's embrace. She attracted ire from the moment her husband attained elected office by condemning the California governor's mansion as "a noisy firetrap" and refusing to live there.

In the White House, she was often viewed as imperious with monarchal tendencies, ordering an $800,000 renovation of the White House with Hollywood designer Ted Graber and the addition of a 4,370-piece red-rimmed, Lenox porcelain service for $210,000. The tableware was donated by close friends, but that hardly muted criticism.

In stark contrast to the simple sartorial style of Rosalynn Carter who preceded her, Mrs. Reagan adored designer clothes, favoring red bouclé suits by Adolfo and the couture finery of Philadelphia-born James Galanos. The donation of clothes by America's top designers led to further criticism, though her crisp elegance rarely hit a misstep.

Her job, always, was to serve her husband.

During the last 10 years of the president's life, Mrs. Reagan rarely left his side, and often appeared weak, wan, and utterly devastated in public.

Maureen Reagan, the president's daughter from his first marriage to Jane Wyman, said in 2000, a year before her death from melanoma: "Talk about rising to the occasion. I can say that as my father has weakened, Nancy has gotten stronger."

In 2002, Mrs. Reagan began quietly, yet forcefully, lobbying congressmen against President George W. Bush's strict limits on stem-cell research. In July 2004, weeks after his father's death at age 93, Ron Reagan addressed the Democratic National Convention championing stem-cell research, a move that suggested his mother's private blessings.

Her championship of her husband and medical research and her constant nursing during his slow decline, raised Mrs. Reagan's reputation, even among staunch critics, coming to overshadow her image as a petite dowager in designer clothes who suggested Americans "just say no" to drugs.

A wrenching childhood

Mrs. Reagan was raised in markedly different economic circumstances from her husband, who came from a humble background, the son of a Dixon, Ill., shoe salesman, though their early years were both marked by emotional abandonment.

She was born Anne Frances Robbins in New York City, July 6, 1921, to an actress mother and car salesman father who separated when she was 2. Nicknamed Nancy, she lived for five years with an aunt in suburban Washington while her mother, Edie, pursued an acting career in New York.

Her mother eventually married Chicago neurosurgeon Loyal Davis, whom she accepted as her true father. Davis was a rigid disciplinarian who raised the family in considerable comfort, sending her to the elite Girls Latin School. She legally changed her name to Nancy Davis at age 16 when he adopted her.

Her young acting career

Nancy Davis graduated from Smith College in 1943, then embarked on a Broadway acting career before moving to Los Angeles in 1949 as a contract player for MGM.

A staunch conservative, Miss Davis discovered her name mistakenly on a list of communist sympathizers and contacted Ronald Reagan, then a Democrat, president of the Screen Actors Guild, and newly divorced from Jane Wyman. The attraction between Miss Davis and Reagan was mutual and immediate.

Michael Reagan once said that his mother, Jane Wyman, gets "lost in her work. Nancy gets lost in Dad."

Nancy and Ronald Reagan wed in March 1952, with only actor William Holden and his wife, Ardis, as attendants. Daughter Patti arrived seven months later. Son Ron was born in 1958.

The marriage was her crowning achievement. On the occasion of their 31st anniversary in 1983, the president wrote Mrs. Reagan, "I more than love you, I'm not whole without you. You are life itself to me. When you are gone I'm waiting for you to return so I can start living again."

Staff writer David O'Reilly contributed to this article.