A year ago this week, Zohran Mamdani’s surprise victory in the Democratic primary for mayor upended New York politics.

Now, in the closing days of another primary season, he has thrown himself back onto the campaign trail, this time risking his political capital in a high-stakes bid to catapult fellow leftists to primary victories against the old Democratic guard.

Mr. Mamdani and allies are attempting to unseat two Democratic incumbents, Representatives Daniel Goldman and Adriano Espaillat, whom they view as too friendly to corporate donors and Israel. They want to lay claim to a third House seat. And down the ballot, they have designs on expanding the socialist block in Albany.

If he prevails on Tuesday, Mr. Mamdani, 34, will go a long way toward establishing socialists as a major faction in New York City politics and himself as a kingmaker capable of vaulting relatively unknown candidates to victory and sidelining erstwhile power brokers.

But a string of losses could be disastrous, weakening the mayor’s political standing just six months into his term, empowering political opponents and creating new ones.

His involvement has already alienated Black and Latino progressives, powerful labor unions and the left-leaning Working Families Party, all of which helped him get to City Hall and partnered with him as mayor. Some, like Representative Nydia Velázquez, have taken the rare step of publicly declaring they have lost trust in him.

“I have a pit in my stomach because of secondhand anxiety,” said Michael Lange, an elections analyst and fellow democratic socialist who rose to prominence chronicling Mr. Mamdani’s ascent.

“This is a way to remake the Democratic Party,” he said. “But if he loses, the knives would be out. They would be really out. The risk is that they’ll say this is more man than movement.”

The mayor’s support goes further than mere endorsements. With his popularity never higher, Mr. Mamdani has personally involved himself in everything from candidate recruitment and fund-raising to ad shoots and private strategy sessions. A pair of his top political aides are helping run two of the campaigns. And the mayor attempted to push labor unions into backing at least one of his candidates.

On Thursday, he headlined a rally at Kings Theater in Brooklyn aimed at recapturing some of the magic of his own come-from-behind victory and the army of volunteers who powered it.

“I know there are some who may ask, is that election not over?” Mr. Mamdani said. “It may be, but our work has never been about any one person, any one office or any one election.”

“The Democratic Party,” he later added, “must change.”

Polls in the final weeks suggest two of the candidates that Mr. Mamdani endorsed — Claire Valdez in New York’s Brooklyn- and Queens-based Seventh District and Darializa Avila Chevalier in the Manhattan- and Bronx-based 13th — are in tossup races.

Brad Lander, a longtime city official with his own committed base, appears to be in a more commanding position to oust Mr. Goldman in the parts of Lower Manhattan and Brooklyn that make up the 10th District.

Mr. Mamdani still has work to do. He succeeded last year by driving a huge number of young voters to the polls, diluting the influence of more moderate older ones. But without him on the ballot, after week of early voting, the electorate appeared to be trending toward being smaller and older.

The trend was pronounced enough that, in recent days, the local chapter of the Democratic Socialists of America called an emergency meeting to discuss what one leader described as the “cratering” youth vote and how to correct course, according to someone who participated in the meeting who asked for anonymity to disclose private details.

Mr. Mamdani spent much of the weekend barnstorming from district to district to help, campaigning shoulder-to-shoulder with his candidates.

But his face could also be seen in paid ads and cable news interviews promoting them now streaming across the city. (Much of it was paid for with funds Mr. Mamdani helped raise in well-appointed living rooms and at star-studded soirees all spring.)

There is no real recent precedent for such an aggressive mayoral posture in off-year elections in New York City. Michael R. Bloomberg and Bill de Blasio got involved in State Senate contests when each was mayor. But modern mayors have tended to want to broaden their political coalitions while governing, not risk shrinking them.

Mr. Mamdani and his allies argue that no one should be surprised that a mayor who stormed into office by ignoring old political conventions and challenging more seasoned candidates would take on more risk now.

In an interview backstage after the rally at the theater, he played down concerns about early turnout and said he was willing to accept collateral damage.

“There will always be risk when it comes to making a decision about how best to fight for working people,” he said, “And I believe that it is worth it when it comes to these candidates.”

Mr. Mamdani had not yet cemented his own victory last fall when he began casting his attention to New York’s next primary season.

In October, he met with Mr. Lander, a mayoral rival turned ally who wanted to join his administration, to encourage him to challenge Mr. Goldman. The incumbent had close ties to the American Israel Public Affairs Committee and in a district that would soon vote for Mr. Mamdani overwhelmingly, he had refused to endorse him. Mr. Lander has denounced the pro-Israel lobby and been far more critical of the country’s war in Gaza.

Tensions really began to rise, though, when Ms. Velázquez, an early Mamdani supporter, announced her retirement after three decades in the district Mr. Mamdani won overwhelmingly.

Ms. Velázquez hoped that one of her protégés (including some who are members of D.S.A.) might succeed her, but Mr. Mamdani batted those possibilities away in favor of Ms. Valdez. She was only in her first term as a state assemblywoman, and Ms. Velázquez barely knew her. But she was an early supporter of his mayoral ambitions and another critic of Israel.

The congresswoman was furious. “I’ve never heard her use more four-letter words,” Letitia James, the state’s attorney general, said.

Ms. Velázquez backed another candidate, the left-leaning Brooklyn borough president, Antonio Reynoso.

The resulting race has driven a wedge between socialists and progressives and exposed painful questions tied to the area’s rapid gentrification. Ms. Velázquez and the Working Families Party have accused the D.S.A. of trying to push them aside while capitalizing on their long-term organizing. The socialists say the progressives are not meeting the moment.

Mr. Mamdani’s support for Ms. Avila Chevalier, another democratic socialist, came in late May. In many ways, it has been the most divisive.

Mr. Espaillat, her opponent, is the chairman of the Congressional Hispanic Caucus and a close ally of Representative Hakeem Jeffries, the New Yorker trying to become the next House speaker. Like other candidates Mr. Mamdani is opposing, Mr. Espaillat has taken progressive stances on most issues.

Mr. Espaillat did not back Mr. Mamdani in last year’s primary, but afterward, he quickly endorsed him. At the time, Mr. Mamdani privately assured Mr. Espaillat he would return the favor if he ever needed it. Yet as Ms. Avila Chevalier gained momentum, Mr. Mamdani saw an opportunity to elevate someone who is a vehement critic of Israel, whereas Mr. Espaillat has been close to AIPAC.

Mr. Jeffries and Latino leaders were furious.

“In politics and in life, you’re as good as your word,” said Fernando Ferrer, the former mayoral nominee and a Latino trailblazer in the city.

For Mr. Jeffries, the primaries are particularly personal: Ms. Valdez and Ms. Avila Chevalier have not committed to backing him in his quest to become the House leader, but their far-left positions could also be turned into cudgels by Republicans to attack more moderate Democrats in swing districts that will decide the majority. (Ms. Avila Chevalier has spent weeks apologizing for inflammatory old tweets, including saying that “all deportations are wrong” and using crude language about Kamala Harris.)

“Every Democrat in a competitive race will have to answer for our most extreme voices,” said Howard Wolfson, political adviser to Mr. Bloomberg and a former head of the Democratic Congressional Campaign Committee. “And it will make the party as a whole seem extreme and out of touch.”

The groups aligned with Mr. Mamdani in Tuesday’s elections view those tensions as necessary costs.

“The reward is greater than the risk,” said Usamah Andrabi, a leader of Justice Democrats, a national group spending to help Ms. Avila Chevalier and Ms. Valdez.

He said he admired Mr. Mamdani for putting his own political capital at risk “to fight for the movement that you think the city deserves.”

Ms. James, who once compared Mr. Mamdani to former President Barack Obama, said she respected the mayor, but respected “the work and dedication” of Ms. Velázquez and Mr. Espaillat “more.”

“I just hope that once we are past this political season that he can make amends and heal the rift,” she said, but added a note of skepticism.

“Some people argue New Yorkers have short memories,” Ms. James said. “I don’t know about that. This one runs deep.”

Benjamin Oreskes contributed reporting.