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 re-election 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.