Micah Lasher, a first-term Democratic state lawmaker from Manhattan, said on Monday that he is running to succeed his political mentor, Representative Jerrold Nadler, who plans to retire next year after three decades in Congress.
Mr. Lasher is among the first to officially enter what is expected to be a crowded primary to represent one of the most Democratic congressional districts in the United States.
In a video announcing his candidacy, Mr. Lasher, a state assemblyman who represents the Upper West Side, outlined policy proposals that included raising the federal minimum wage and passing “Medicare for All.”
But he said his top priority would be fighting what he called the Trump administration’s assault on the Constitution, democratic norms and vital public programs.
“I think the fundamental question facing Congress today is: How do we muster every ounce of power we can to stop the Trump regime and their reactionary program?” Mr. Lasher, 43, said in an interview.
His entry into the race comes two weeks after Mr. Nadler, at 78 one of Congress’s leading liberal voices, said he would step down when his current term ended, heeding calls within his party for generational change.
That decision has created a rare opening in the 12th Congressional District. Running from the East River to the Hudson and from Union Square to the top of Central Park, the district is a bastion of a kind of Jewish liberalism that is closely linked with New York City Democratic politics.
Even before Mr. Nadler’s announcement, one younger Democrat, Liam Elkind, had begun a primary campaign for the seat. Since then, several others — including Erik Bottcher, a City Council member from the West Side; Alex Bores, a state assemblyman from the East Side; and Jack Schlossberg, a grandson of President John F. Kennedy — have indicated they are considering running.
Mr. Nadler remains popular in the district and his blessing would be significant in the primary. He has not endorsed a potential successor, but a person familiar with his thinking said that Mr. Nadler would most likely support Mr. Lasher, his protégé and onetime aide, as the race proceeds.
Although Mr. Lasher has been an elected official for only a year, he has a long résumé in politics and government.
He worked on his first campaign at 16, helping to elect Eric Schneiderman, a fellow West Sider, to the State Senate. By 17, he was an informal adviser to Scott M. Stringer in the State Assembly. He helped start the political consulting firm SKDKnickerbocker (now called SKDK) before graduating from college.
Since then, he has held some of the most influential unelected roles in city and state government, including legislative director for Mayor Michael R. Bloomberg and policy director for Gov. Kathy Hochul.
Mr. Lasher describes himself as a “practical progressive” who is committed to working across the political spectrum. Even so, he is willing to take hard-line partisan positions. He is leading the charge to let New York engage in mid-cycle redistricting to counter the actions of Texas Republicans. And he argues that congressional Democrats should be willing to shut down the government rather than fund what he calls the Trump administration’s “fascist policies.”
The seat he seeks is one in which Mr. Nadler has established himself as a pro-Israel progressive, a position that has become more tenuous amid the war in Gaza and the Democratic Party’s divisions along ideological, economic and generational lines.
In New York City, those divisions have come to life in this year’s mayoral race. In the June primary, Mr. Nadler’s district was nearly evenly split between Zohran Mamdani, a Queens assemblyman and democratic socialist, and former Gov. Andrew M. Cuomo, a moderate.
Mr. Nadler, who had long navigated his party’s divergent factions, supported Mr. Stringer in the primary but switched to Mr. Mamdani after he became the nominee, noting the groundswell of enthusiasm the 33-year-old assemblyman had tapped into on his way to victory.
A few days later, Mr. Lasher endorsed Mr. Mamdani in a lengthy social media post. He made it clear that he had wrestled with his “deeply felt disagreements” with the nominee, including about Israel’s status as a Jewish state (Mr. Mamdani has said he believes Israel should provide equal rights for all its residents).
But Mr. Lasher also cited Mr. Mamdani’s commitment to engaging seriously with people he disagrees with, and pledged both to hold him to account if he became mayor and to do all he could to help him be the best leader he could be.
“I don’t insist on unanimous agreement to find ways of working with people of good will,” Mr. Lasher said in an interview.
Mr. Lasher’s one area of relative inexperience in the public sphere may be in the humbling and exhilarating role of candidate for elected office. In the interview, he admitted that the transition to moving out front from behind the scenes had been challenging.
“It is not infrequently terrifying,” he said, deadly serious. “And occasionally excitement creeps in.”