Zohran Mamdani, the Democratic nominee for mayor of New York City, received the backing of significant party figures this week as Democrats grow increasingly concerned about President Trump’s efforts to meddle in the race.

Representative Jamie Raskin of Maryland and Representative Pat Ryan of New York offered Mr. Mamdani, a state assemblyman and democratic socialist, their formal endorsement. Rahm Emanuel, the former mayor of Chicago, and Representative Ritchie Torres, two moderate Democrats with close ties to the Jewish leaders who are skeptical of Mr. Mamdani, praised his potential to effectively lead the city.

Top Democrats have treaded cautiously for months after Mr. Mamdani’s decisive primary victory in June over former Gov. Andrew Cuomo, waiting weeks to even meet with their party’s nominee. The New York Democrats who have so far declined to endorse his bid are notable in their absence from his camp: Representative Hakeem Jeffries, the House Democratic leader; Senator Chuck Schumer, the minority leader; Senator Kirsten Gillibrand; and Gov. Kathy Hochul.

Now, publicly and privately, Democrats across the ideological spectrum are arguing that it is important for their leaders to forcefully reject Mr. Trump’s apparent interest in tipping the race toward Mr. Cuomo, who is running as an independent.

“In these times, the Democratic Party needs to stick together with the maximum solidarity and focus,” Mr. Raskin, who has taken a leading role in the party’s efforts to oppose Mr. Trump, said in an interview. “Even though I’m not a New Yorker and have never been a New Yorker, I feel that Democrats must stand together to defend not only our party but our constitution and our country.”

Mr. Raskin, the top Democrat on the House Judiciary Committee, praised Mr. Mamdani as a “significant and inspiring leader.”

“Mamdani’s politics have a Rooseveltian quality to them,” he said, comparing the 33-year-old politician to Franklin D. Roosevelt, a Democratic president who built a lasting political coalition by enacting transformative liberal legislation. “He really wants to rebuild an F.D.R. coalition that is fundamentally committed to the success of the working and middle classes in his city.”

Mr. Mamdani, who ran a campaign focused tightly on cost-of-living issues, quickly racked up endorsements from the left wing of his party. Senators Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts backed his bid, as did Representatives Pramila Jayapal of Washington and Alexandria Ocasio-Cortez and Jerrold Nadler of New York.

Liberals who are frequently pressured to quickly coalesce behind more moderate nominees have made little secret of their irritation with the party’s establishment for refusing to endorse Mr. Mamdani.

“I think that it is the responsibility of our party to come together after our primary elections and support our nominees,” Ms. Ocasio-Cortez said on Monday. “I have long held that position, regardless of our disagreements, that’s what our primary elections are for.”

But Mr. Trump’s extraordinary efforts to shape the race may be shifting the calculus. As the race moves into the fall stretch, there’s frustration among some Democrats on Capitol Hill, including Senator Chris Van Hollen of Maryland and Mr. Raskin, that the New York federal delegation has not fully backed Mr. Mamdani.

Advisers to Mr. Trump have attempted to create a one-on-one contest between Mr. Cuomo and Mr. Mamdani by incentivizing Mayor Eric Adams and the Republican candidate, Curtis Sliwa, to end their campaigns by offering them positions in his administration. In recent months, Mr. Trump has railed against Mr. Mamdani and discussed with associates how to help set up a rematch with Mr. Cuomo.

Mr. Ryan, the New York Democrat who endorsed Mr. Mamdani on Wednesday, said he was largely motivated by his dislike of Mr. Cuomo, whom he described as “a corrupt person.”

“In this one, it was very, very crystal clear who’s for the people and who is for themselves,” said Mr. Ryan, who represents what has historically been a swing district north of the city. “And if anybody had any doubt about that, Donald Trump’s maneuvering validated that Cuomo would sell his soul to the devil easily.”

Some moderate Democrats worry that Mr. Mamdani could hurt their party in the midterm elections next year both by encouraging more left-wing candidates and by giving Republicans fodder to paint the entire party as aligned with socialism.

Representatives Laura Gillen and Tom Suozzi, who represent competitive districts on Long Island, have aggressively distanced themselves from Mr. Mamdani.

“I believe the future of our party is that we need to lead from the middle because that’s where the majority of Americans are,” said Ms. Gillen, who says the party’s leadership should not back Mr. Mamdani’s bid. “I’m certainly not endorsing him.”

Still, the refusal of Democratic leaders to back Mr. Mamdani is baffling to some of his liberal supporters, who say the state assemblyman is inspiring Democrats across the country at a moment when their party’s approval rating is at record lows.

Polling conducted this week by The New York Times and Siena University showed Mr. Mamdani with a commanding lead in the race, buoyed by particularly strong support among young voters. That group, long a core part of the Democratic coalition, swung significantly to Mr. Trump in the 2024 election.

“I don’t understand why it’s even a question. Of course, they should endorse him,” said Representative Ro Khanna, a liberal Democrat from California.

For his part, Mr. Mamdani has been on a charm offensive of private meetings and phone calls since capturing the nomination.

At a recent breakfast meeting with Mr. Emanuel, the moderate former congressman from Illinois, mayor of Chicago and ambassador, Mr. Mamdani asked for advice about prioritizing his policy goals, governing a major city and staffing his administration.

“This guy is thinking about how to hit the ground running and he showed the full capacity to do that from his questions,” said Mr. Emanuel. “I don’t think he’s going to spend four years just throwing rhetorical bombs.”

Mr. Emanuel, who is broadly disliked by his party’s liberal wing, said he opened the breakfast with a joke about how Mr. Mamdani has divided Democrats. One of his former campaign aides just went to work as Mr. Cuomo’s campaign manager.

“I said to him: ‘Well, who’s going to hate this more, my rabbi or A.O.C.,” he recounted, saying Mr. Mamdani responded with a hearty laugh.

The day after capturing the nomination, Mr. Mamdani had a lengthy conversation with former President Barack Obama, who offered advice on navigating the new political and governing moment Mr. Mamdani found himself facing, according to a person familiar with the conversation.

Mr. Torres, a Democrat from the Bronx who represents a large modern Orthodox Jewish community, also met with Mr. Mamdani shortly after the primary. While he hasn’t formally endorsed his bid, Mr. Torres has repeatedly praised Mr. Mamdani and defended him from Republican attacks.

“He is as impressive as any person I’ve ever met in politics,” said Mr. Torres, who is one of the party’s strongest supporters of Israel. “He’s impressively knowledgeable and substantive on public policy. Even when you disagree with him there’s no denying the force of his intellect.”

Party leaders and most of the New York delegation have not been as welcoming. After weeks of discussion, Mr. Schumer met with Mr. Mamdani at his New York City office on Monday but declined to endorse his bid.

“We know each other well, and we’re going to keep talking,” he told reporters.

Mr. Jeffries hinted that he was moving toward a decision when asked by reporters on Capitol Hill on Wednesday whether he planned to back his party’s nominee in his home city. The two men, who met twice in Brooklyn over the summer, do not currently have any additional meetings scheduled.

“I certainly will have more to say about the New York City mayor’s race in short order,” he told reporters.

Mr. Jeffries previously said he had concerns about Mr. Mamdani’s refusal to condemn the term “globalize the intifada” — a position Mr. Mamdani softened this past weekend.

Ms. Hochul said in an interview on Bloomberg TV on Wednesday that she is having “conversations about all kinds of issues” with Mr. Mamdani, adding that she is a “staunch capitalist.”

“I need to know that people can have a certain philosophy, but you got to govern in reality,” she said. “And the reality is that this is the financial center of the world.”

Republicans have gleefully seized on Mr. Mamdani as a foil. The National Republican Campaign Committee has given Mr. Mamdani a starring role in dozens of videos, news releases and social media posts attacking Democratic candidates.

“We will do whatever we have to defend people against a communist mayor out of state,” said Gov. Ron DeSantis of Florida in an interview on Fox News in July. “We’re not going to let him come and spew his leftism in the free state of Florida.”

Those attacks don’t particularly concern Mr. Raskin, who sees Democratic solidarity as more important.

“When moderates beat progressives, we want the progressives to support the moderates in the general election. And when progressives beat moderates in the primary, we want the moderates to support the progressives in the general election,” he said. “I’m a true blue Democrat. I support Democrats across the political spectrum.”

Patricia Mazzei and Megan Mineiro contributed reporting.