California Sen. Dianne Feinstein, a pioneer for women in politics and a stalwart of the U.S. Senate for more than three decades, has died at 90, her office confirmed.

"Her passing is a great loss for so many, from those who loved and cared for her to the people of California that she dedicated her life to serving," James Sauls, Feinstein's chief of staff, wrote in a statement Friday morning. 

"Senator Feinstein was a force of nature who made an incredible impact on our country and her home state," Sauls wrote. "She left a legacy that is undeniable and extraordinary."

Feinstein was the longest-tenured female senator in U.S. history and, prior to her death, was the oldest sitting member of Congress. She was first elected to represent California in 1992 and has since been reelected five times, including in 2012, when she received a record 7.86 million votes, the highest popular vote total in U.S. Senate history.

Throughout her career, she has been a trailblazer for women in politics, becoming the first woman to chair the Senate's all-important Rules and Intelligence Committees, as well as the first to oversee a presidential inauguration for Barack Obama in 2009. She was also the first woman to serve as the president of the San Francisco Board of Supervisors and the first woman to lead the city as its mayor.

"I'm deeply saddened by the passing of Dianne Feinstein," wrote Hillary Clinton, the first woman elected to serve as a major party's presidential nominee. "She blazed trails for women in politics and found a life's calling in public service. I'll miss her greatly as a friend and colleague and send my condolences to all who loved her."

Former House Speaker Nancy Pelosi, another prominent San Franciscan and trailblazing California Democrat, offered a bereavement resolution and led a moment of silence on the floor of Congress in honor of Feinstein on Friday.

"Her indomitable, indefatigable leadership made a magnificent difference for our national security and personal safety, the health of our people and our planet, and the strength of our Democracy," Pelosi said of Feinstein in a separate statement, later adding: "Dianne’s extraordinary career will continue to inspire countless young women and girls to pursue public service for generations to come." 

She was also the first female Jewish senator, and prior to her death was one of just 10 Jewish members of the U.S. Senate.

"Earlier this morning, we lost a giant in the Senate," Majority Leader Chuck Schumer, D-N.Y., said on the Senate floor after leading a moment of silence for Feinstein, with a black cloth and bowl of white flowers draped over her desk in the chamber. "Sen. Dianne Feinstein was one of the most amazing people who ever graced the Senate, who ever graced the country."

"As the nation mourns this tremendous loss, we're comforted in knowing how many mountains Dianne moved, how many lives she impacted, how many glass ceilings she shattered along the way," Schumer later added. "America, America is a better place because of Sen. Dianne Feinstein."

Feinstein rose to prominence when she took over as mayor following the assassinations of San Francisco Mayor George Moscone and San Francisco Supervisor Harvey Milk, the first openly gay man to be elected to public office in California, by a disgruntled former supervisor. Feinstein was the one who found Milk's body, recounting that her finger slipped through a bullet hole as she felt for a pulse.

Feinstein was a vocal gun safety advocate, authoring the 1994 Assault Weapons Ban and pushed for years to enact new safety measures after the ban expired in 2004. 

"From 1994, when she passed the Assault Weapons Ban as a first term Senator, until the tragedy in Newtown, Dianne Feinstein was a lonely voice fighting against gun violence," Connecticut Sen. Chris Murphy, a Democrat and staunch guns afety advocate, wrote on social media on Friday. "The modern anti-gun violence movement, now stronger than the gun lobby, would not exist but for Dianne."

"Senator Dianne Feinstein was a champion for Gun Violence Prevention that broke barriers at all levels of government," posted Florida Rep. Maxwell Frost, who previously served as national organizing director for gun control group March for Our Lives. "We wouldn’t have had an assault weapons ban if it wasn’t for Senator Feinstein and due to her tireless work, we will win it back. May her memory be a blessing."

Known as a centrist, Feinstein worked closely with her Republican colleagues, frequently crossing the aisle to try and find consensus on key issues – and often drawing the ire of her more liberal colleagues. But her bipartisan nature won her praise and admiration from both sides of the aisle.

Senate Republican leader Mitch McConnell hailed Feinstein a "trailblazer" and an "actual friend," adding that "our entire nation [is] better for her dogged advocacy and diligent service" in remarks on Friday. And House Speaker Kevin McCarthy, R-Calif., hailed her as someone who "inspired women from both sides of the aisle to seek elected office."

"Senator Dianne Feinstein was a trailblazer who lived an incredible life dedicated to public service," North Carolina Sen. Thom Tillis, a Republican, said Friday. "She was one of the most effective legislators in recent memory because of her willingness to work across the aisle in good faith in order to solve complex problems. It was a honor to serve with her. Susan and I extend our deepest condolences and prayers to Senator Feinstein’s family and staff during this difficult time."

Earlier this year she announced she would not seek reelection to the seat she has held since 1992 amid concerns about her health and questions about her fitness to serve. Such treatment was condemned by Pennsylvania Sen. John Fetterman, who slammed those who were "piling on" the California senator, pointing out her moments of confusion and highlighted "unflattering pictures" of her.

"Let's make sure that there should be dignity for those kinds of individuals," he told reporters at the Capitol on Friday.

Regardless, she rebuffed calls to resign – even as her absences from the Senate piled up in recent months. She was missing form the Senate for more than two months earlier this year as she recovered from a bout of shingles. 

Feinstein worked in the Senate up until the day before she died. She cast a vote on Thursday to advance the Senate's short-term funding bill to avoid a government shutdown.

In a statement on Friday morning, President Joe Biden, who served alongside Feinstein in the Senate for decades as a Delaware senator and Vice President, called her a "pioneering American," a "true trailblazer" and "a cherished friend" to himself and first lady Jill Biden.

"In San Francisco, she showed enormous poise and courage in the wake of tragedy, and became a powerful voice for American values," the president wrote. "Serving in the Senate together for more than 15 years, I had a front row seat to what Dianne was able to accomplish. It’s why I recruited her to serve on the Judiciary Committee when I was Chairman – I knew what she was made of, and I wanted her on our team."

Biden hailed her actions to ban assault weapons and said the longtime lawmaker "made her mark" on issues ranging from national security to protecting the environment.

"Often the only woman in the room, Dianne was a role model for so many Americans – a job she took seriously by mentoring countless public servants, many of whom now serve in my Administration," he said. "She had an immense impact on younger female leaders for whom she generously opened doors. Dianne was tough, sharp, always prepared, and never pulled a punch, but she was also a kind and loyal friend, and that’s what Jill and I will miss the most."

The onus is now on Gov. Gavin Newsom to put forward her replacement amid an already crowded primary to replace her. In a social media post on Friday morning, Newsom called her "a dear friend, a lifelong mentor, and a role model not only for me, but to my wife and daughters for what a powerful, effective leader looks like."

"Every race she won, she made history, but her story wasn’t just about being the first woman in a particular political office, it was what she did for California, and for America, with that power once she earned it," Newsom wrote. "That’s what she should be remembered for. There is simply nobody who possessed the strength, gravitas, and fierceness of Dianne Feinstein."

A trio of prominent House Democrats – Reps. Adam Schiff, Barbara Lee and Katie Porter – are currently vying for the seat Feinstein held, putting pressure on the Democratic governor to install an interim appointment.

"Senator Feinstein was a trailblazer for women in California politics, and her leadership on gun violence prevention and anti-torture made our nation more just," Porter wrote on social media. "I wish her loved ones strength during this difficult time."

"California lost a trailblazer for women and a political giant today," Lee wrote, calling her a "friend and mentor" and a "fearless trailblazer with an iron backbone who led the charge on gun control and never stood down from her demands of transparency and accountability in American government."

"Her legacy and impact have forever shaped our nation," Lee added. "This is a monumental day of loss."

In a recent interview with NBC News’ “Meet the Press,” Newsom said he did not want to tip the scales for any of the candidates currently running.

“I don’t want to get involved in the primary,” Newsom said at the time. “It would be unfair to the Democrats that have worked their tail off. That primary is just a matter of months away. I don’t want to tip the balance of that.”

Newsom also said he would keep his pledge to appoint a Black woman to the seat. There have been no Black women in the Senate since Kamala Harris' departure in 2021 to serve as vice president, the Senate has lacked any Black women.

"For years, I witnessed Senator Feinstein's leadership, when the cameras were on and when they were off," Harris said of the longtime lawmaker in a statement, adding: "Senator Feinstein and I shared a fundamental belief in the importance of strong American leadership. And I saw firsthand how she worked courageously to ensure that our leadership was guided by our nation’s values."

"In the tradition of so many great Senators from California, she was not only a leader for our state, but for our nation and our world," she continued. "Through her long career, Senator Feinstein worked across the aisle to help our nation live up to its promise."

Newsom appointed Alex Padilla, then-California's secretary of state, to fill the vacancy left behind by Harris, making him the first Latino senator to represent California in the Senate.

"Long before serving together in the Senate, Dianne gave me one of my first jobs in politics in her Los Angeles office at the start of my career as I was looking to make a difference in California," Padilla said in a statement. "It’s in part thanks to her groundbreaking career that a Latino son of immigrants could one day not just work for her, but work alongside her in fighting to keep the American Dream alive."

Lee fired back at Newsom over his comments earlier this month about Feinstein's eventual replacement, saying at the time that "the perspective of Black women in the U.S. Senate is sorely needed."

"If the Governor intends to keep his promise and appoint a Black woman to the Senate, the people of California deserve the best possible person for that job," Lee said in a statement. "Not a token appointment."