Representative Jerrold Nadler, a New York Democrat who has been one of Congress’s leading liberal voices for three decades, will not seek re-election next year, heeding a call for generational change roiling his party.

The decision will mark the close of a 34-year congressional career that put Mr. Nadler at the center of major civil rights battles and three presidential impeachments. It will also almost certainly touch off a crowded primary fight over a rare open Democratic seat in the heart of Manhattan.

In a recent interview in his downtown Manhattan office, Mr. Nadler, 78, said he hesitated to step aside when he believes that President Trump is threatening the foundations of democracy. But he said he had been persuaded it was time for a changing of the guard.

“Watching the Biden thing really said something about the necessity for generational change in the party, and I think I want to respect that,” Mr. Nadler said, adding that a younger successor “can maybe do better, can maybe help us more.”

Mr. Nadler, who is both the longest-serving New Yorker and Jewish member of the House, had already been swept up in an intraparty reckoning over aging leaders. He was forced to give up his House Judiciary Committee leadership at the beginning of the term when it became clear a younger, more energetic colleague would beat him. Back home, he was facing a 26-year-old primary challenger.

Mr. Nadler shared his plans with The New York Times last week on the condition that they not be publicized until Monday. He spoke for an hour about triumphs, fears and his shifting views on the war in Gaza, all while surrounded by talismans of a half-century in public life: old maps of Manhattan, placards from 16 years in the State Assembly, a photo with Bella Abzug, a former congresswoman and a political mentor.