Nancy Reagan, the influential and stylish wife of the 40th president of the United States who unabashedly put Ronald Reagan at the center of her life but who became a political figure in her own right, died on Sunday at her home in Los Angeles. She was 94.

The cause was congestive heart failure, according to a statement from Joanne Drake, a spokeswoman for Mrs. Reagan.

Mrs. Reagan was a fierce guardian of her husband’s image, sometimes at the expense of her own, and during Mr. Reagan’s improbable climb from a Hollywood acting career to the governorship of California and ultimately the White House, she was a trusted adviser.

“Without Nancy, there would have been no Governor Reagan, no President Reagan,” said Michael K. Deaver, the longtime aide and close friend of the Reagans who died in 2007.

Mrs. Reagan helped hire and fire the political consultants who ran her husband’s near-miss campaign for the Republican presidential nomination in 1976 and his successful campaign for the presidency in 1980. She played a seminal role in the 1987 ouster of the White House chief of staff, Donald T. Regan, whom Mrs. Reagan blamed for ineptness after it was disclosed that Mr. Reagan had secretly approved arms sales to Iran.

Behind the scenes, Mrs. Reagan was the prime mover in Mr. Reagan’s efforts to recover from the scandal, which was known as Iran-contra because some of the proceeds from the sale had been diverted to the contras opposing the leftist government of Nicaragua. While trying to persuade her stubborn husband to apologize for the arms deal, Mrs. Reagan brought political figures into the White House, among them the Democratic power broker Robert S. Strauss, to argue her case to the president.

Mr. Reagan eventually conceded that she was right. On March 4, 1987, the president made a distanced apology for the arms sale in a nationally televised address that dramatically improved his slumping public approval ratings.

His wife, typically, neither sought nor received credit for the turnaround. Mrs. Reagan did not wish to detract from her husband’s luster by appearing to be a power behind the presidential throne.

In public, she gazed at him adoringly and portrayed herself as a contented wife who had willingly given up a Hollywood acting career of her own to devote herself to her husband’s career. “He was all I had ever wanted in a man, and more,” she wrote in “My Turn: The Memoirs of Nancy Reagan,” published in 1989.

He reciprocated in kind. “How do you describe coming into a warm room from out of the cold?” he once said. “Never waking up bored? The only thing wrong is, she’s made a coward out of me. Whenever she’s out of sight, I’m a worrier about her.”

In truth, she was the worrier. Mrs. Reagan wrote in her memoirs that she sometimes became angry with her husband because of his relentless optimism. He didn’t worry at all, she wrote, “and I seem to do the worrying for both of us.”

It was this conviction that led Mrs. Reagan to take a leading role in the Regan ouster and in other personnel matters in the White House. “It’s hard to envision Ronnie as being a bad guy,” she said in a 1989 interview. “And he’s not. But there are times when somebody has to step in and say something. And I’ve had to do that sometimes — often.”

She did not always get her way. Mr. Reagan ignored her criticism of several cabinet appointees, including Defense Secretary Caspar W. Weinberger.

In 2001, seven years after her husband announced that he had Alzheimer’s disease, Mrs. Reagan broke with President George W. Bush and endorsed embryonic stem cell research. She stepped up her advocacy after her husband’s death on June 5, 2004. “She feels the greatest legacy her family could ever have is to spare other families from going through what they have,” a family friend, Doug Wick, quoted Mrs. Reagan as saying.

Years on Camera

Born Anne Frances Robbins on July 6, 1921, in New York City, Nancy Davis was the daughter of Edith Luckett, an actress, and Kenneth Robbins, a car dealer who abandoned the family soon after her birth. Ms. Luckett resumed her stage career when her daughter was 2 and sent the child to live with relatives in Bethesda, Md. In 1929, Ms. Luckett married a Chicago neurosurgeon, Loyal Davis, who adopted Nancy and gave her the family name.

Almost overnight, Nancy Davis’s difficult childhood became stable and privileged. Throughout the rest of her life, she described Mr. Davis as her real father.

Nancy Davis graduated from the elite Girls’ Latin School in Chicago and then from Smith College in 1943. Slender, with photogenic beauty and large, luminous eyes, she considered an acting career. After doing summer stock in New England, she landed a part in the Broadway musical “Lute Song,” with Mary Martin and Yul Brynner. With the help of a friend, the actor Spencer Tracy, her mother then arranged a screen test given by the director George Cukor, of MGM.

Cukor, according to his biographer, told the studio that Miss Davis lacked talent. Nonetheless, she was given a part in the film she had tested for, “East Side, West Side,” which was released in 1949 starring Barbara Stanwyck, James Mason and Ava Gardner. Cast as the socialite wife of a New York press baron, Miss Davis appeared in only two scenes, but they were with Miss Stanwyck, the film’s top star.

After her husband went into politics, Mrs. Reagan encouraged the notion that her acting interest had been secondary, a view underscored by the biographical information she supplied to MGM in 1949, in which she said her “greatest ambition” was to have a “successful, happy marriage.”

But this was a convention in a day when women were not encouraged to have careers outside the home. In his book “Reagan’s America: Innocents At Home,” Garry Wills disputed the prevalent view that Miss Davis had just been marking time in Hollywood while waiting for a man. She was “the steady woman,” he wrote, who in most of her 11 films had held her own with accomplished actors.

The producer Dore Schary cast Miss Davis in her first lead role, in “The Next Voice You Hear” (1950), playing a pregnant mother opposite James Whitmore. She received good reviews for her work in “Night Into Morning” (1951), with Ray Milland, in which she played a war widow who talked Milland’s character out of committing suicide. Mrs. Reagan thought this was her best film.

Mr. Wills wrote that she was underrated as an actress because she had become most widely associated with her “worst” and, as it happened, last film, “Hellcats of the Navy” (1957), in which Ronald Reagan had the leading role.