SAN FRANCISCO — Nancy Pelosi, the first female speaker of the House and one of the most powerful women in modern American politics, will retire from Congress next year.

The San Francisco Democrat, who announced her decision Thursday, had come under increasing pressure to step aside amid her party’s widening generational divide. Now 85, she defied political observers in 2023 when she ran for a 20th congressional term despite Democrats losing the House and her speakership the year before. But next year, she faced a tougher-than-expected group of primary challengers.

“I will not be seeking reelection to Congress. With a grateful heart, I look forward to my final year of service as your proud representative,” Pelosi said in a lengthy and emotional video posted on social media and addressed to San Franciscans.

One of the most formidable political tacticians of her generation, Pelosi had deflected speculation for months about her plans. Her announcement came two days after California voters overwhelmingly approved a measure to redraw the state’s congressional map to favor Democrats in next year’s midterm elections.

The question of Pelosi’s retirement timeline has for years been the subject of feverish speculation in San Francisco, a deep-blue and politics-obsessed city that has produced a stream of national Democratic leaders, including Gov. Gavin Newsom and former Vice President Kamala Harris. But Pelosi had hinted that she would make a major announcement after Tuesday’s election.

Now, the departure of one of the nation’s most recognizable Democrats, who was first elected to Congress in a 1987 special election, marks the end of an era. It also sets off a cascade of campaign maneuvering here as Democrats seize on the once-in-a-generation opportunity of an open House seat in this center of Democratic politics. And in Washington, Pelosi’s coming departure will prompt questions about the political futures of her senior-ranking congressional allies.

Pelosi already faced two serious Democratic challengers: Saikat Chakrabarti, a wealthy former tech executive and progressive organizer, and Scott Wiener, a prominent state lawmaker who has been angling to succeed Pelosi for years.

Another potential contender for the seat is San Francisco Supervisor Connie Chan, a progressive force inside City Hall who, much like Pelosi, is closely aligned with the city’s powerful labor movement. Pelosi has recently heaped attention on Chan, who’s been featured alongside the former speaker at several recent campaign events.

Pelosi’s departure will deprive Republicans of a chief antagonist they have used as a visceral foil since long before the Trump era — part of a decades-long effort by the GOP to cast Pelosi as a caricature of coastal elite liberalism.

Pelosi, while fading from center stage after stepping aside from her leadership post, continued to wield clout in party politics. She campaigned across the state this fall in support of Democrats’ redistricting measure and was central to the effort to resist President Donald Trump’s cuts to Medicaid and federal safety-net programs. Last year, Pelosi moved behind the scenes to help convince former President Joe Biden not to run for another term. And she’s continued to be a sounding board for Hill Democrats, including her successor, Hakeem Jeffries.

Some Pelosi allies said in recent days that they hoped she would run again, predicting she could help boost Democrats in the midterms.

“She’s back in her five-inch heels. This is not someone who gets lost in the hallways,” said Oakland Rep. Lateefah Simon, a first-term member who counts Pelosi as a mentor and described her as a force at the Capitol even after leaving leadership and recovering from an injury last year. “She has been one of the most consequential legislators in American history. That loss is going to be deeply felt.”

Pelosi has often said she meant to retire years ago but stayed on Capitol Hill to defend the Affordable Care Act — one of her crowning achievements as speaker — amid threats from Republicans. Then, she found herself embroiled in her party’s yearslong power struggle with Trump, who she, like many Democrats, cast as a threat to democracy.

Her farewell video, posted on social media, recalls highlights of Pelosi’s career, including leading efforts to rebuild in San Francisco after the 1989 Loma Prieta earthquake, responding to the HIV/AIDS epidemic, helping to create the city’s Presidio national park site and her trailblazing status as the first female speaker.

“I have truly loved serving as your voice in Congress, and I’ve always honored the song of Saint Francis, ‘Lord make me an instrument of thy peace,’ the anthem of our city,” Pelosi said. “That is why I want you, my fellow San Franciscans, to be the first to know.”

Pelosi also faced personal struggles in recent years that fueled speculation about her political future. Just before the 2022 midterms, David DePape broke into the Pelosis’ San Francisco mansion and attacked Paul Pelosi with a hammer. He was sentenced to life in prison on a slew of state and federal charges. And last year, she underwent hip replacement surgery after suffering an injury while traveling.

Pelosi, in a 2022 documentary by her daughter, filmmaker Alexandra Pelosi, reflected on the accomplishments and travails of her speakership — offering a sobering yet unapologetic take.

“Being speaker makes you a target — a target of misinformation, a target of mockery and sometimes a target of violence,” Pelosi said, before quoting from a prayer she once saw in Africa: “When one day I meet my maker … he will say to me: ‘Show me your wounds.’ … I’m proud of my wounds.”