Two days before New York City Democrats went to the polls to select their mayoral nominee in June, a plane flew over the Statue of Liberty trailing a banner attacking the race’s front-runner, Zohran Mamdani.

“Save NYC from global intifada,” it read in letters five feet high. “Reject Mamdani.”

The banner, seemingly aimed at the city’s Jewish voters, touched on the campaign’s most charged foreign policy issue: Mr. Mamdani’s criticism of Israel. But the group behind it wasn’t Jewish or Israeli. Its members are Indian-American Hindus, who accuse Mr. Mamdani of pushing an anti-Hindu and anti-Indian agenda.

For years, Mr. Mamdani, a Muslim, has assailed the government of Prime Minister Narendra Modi of India, a populist whose political ideology inextricably links nationalism with Hinduism at the expense of the country’s Muslim minority.

Mr. Mamdani in May called the prime minister a “war criminal.” Previously, he lobbied to stop Mr. Modi from visiting New York, and demanded that a state assemblywoman return campaign contributions from Indian Americans whom he characterized as “Hindu fascists.”

While campaigning for the State Assembly in 2020, Mr. Mamdani attended a demonstration in Times Square at which a group protesting the construction of a Hindu temple on the site of a onetime mosque in India chanted, “Who are Hindus? Bastards!”

Mr. Mamdani has never publicly condemned those remarks, and his campaign declined to comment when asked about them.

Now, Mr. Mamdani is within striking distance of becoming the city’s first Muslim and first person of South Asian heritage to become mayor, and he finds himself on the receiving end of attacks by an army of Modi supporters, both in India and the United States.

The efforts reveal how sectarian politics in Delhi can affect an election in New York. In India, attacks on Mr. Mamdani blare from pro-Modi news outlets across millions of TVs and smartphones. In the United States, Indian American groups, some with direct ties to Mr. Modi and his governing Bharatiya Janata Party, are taking a more subtle approach — raising money for Mr. Mamdani’s opponents.

“There were simultaneous campaigns by India-based Hindu nationalists and U.S.-based Hindu groups, pushing the idea that he would be an anti-Hindu candidate,” said Raqib Naik, the director of the Center for the Study of Organized Hate, a watchdog group that tracks Islamophobia online.

Despite such attacks, Mr. Mamdani has found passionate support among many South Asians in New York. Younger, working-class, Muslim and liberal South Asians are energized by the possibility that New York could have its first South Asian mayor, even if some Indian Americans think his views are anti-Hindu. Mr. Mamdani won by large margins in some neighborhoods with sizable South Asian populations, and he captured 52 percent of all first-choice votes cast in majority-Asian neighborhoods.

New York City’s roughly 447,064 South Asian residents are an important pool of voters, but they are hardly a monolithic bloc, and it is hard to predict the effect Hindu opposition might have on Mr. Mamdani’s electoral chances. (Between 2023 and 2024, the Pew Research Center estimated that Hindus make up 2 percent of New York’s population, or about 80,000 people.)

New York is America’s largest city and financial capital, affording its mayor an outsized role on the world stage. As Mr. Mamdani prepares for the general election and tries to reassure moderate Democrats that he is a viable contender, he is also facing the ire of a party machine a world away.

The anti-Mamdani message comes from the top. A B.J.P. national spokeswoman, Sanju Verma, recently called Mr. Mamdani a “Hinduphobic bigot” and “a rabid liar.”

Those talking points have been amplified to a global audience by a network of pro-Modi outlets and influencers.

One anchor at a pro-Modi television network recently called Mr. Mamdani a “part-time revolutionary, full-time Modi baiter.” Another announcer said Mr. Mamdani associated “with Pakistani lobbyists in the United States,” suggesting that the candidate would promote the interests of India’s longtime rival.

According to data reviewed by The Times and compiled by The Center for the Study of Organized Hate, in the weeks between June 13 and June 30, more than 600 posts criticizing Mr. Mamdani were uploaded to X from known right-wing accounts in India and global profiles associated with Hindu nationalism, some with hundreds of thousands of followers.

Rohit Chopra, a professor at Santa Clara University who studies Hindu nationalism, said Mr. Mamdani’s Muslim identity and sharp criticism of Mr. Modi have been enough to “discredit him among Indian Americans while also being played to maximum effect for political capital within India.”

His parents’ backgrounds have provided additional ammunition for the Hindu right. Mr. Mamdani, 33, was born in Uganda, but his father is from Gujarat, Mr. Modi’s home state, where hundreds of people, many of them Muslim, were killed in riots in 2002. His mother, the filmmaker Mira Nair, is a doyenne of India’s progressive left.

In the United States, the distrust of Mr. Mamdani among some Hindus has motivated on-the-ground political action, and those efforts have been supercharged by diaspora groups’ close ties to Mr. Modi and his party.

The Gujarati Samaj of New York, an Indian cultural center in Queens with about 4,000 members, exemplifies the U.S. groups that are in close contact with Mr. Modi and are actively supporting Mr. Mamdani’s opponents.

Samaj members visited Mr. Modi in India in February and remain “in touch” with him, speaking to him “directly,” the group’s president, Harshad Patel, said. In July, members held a fund-raiser for Mayor Eric Adams, the incumbent running against Mr. Mamdani as an independent. In 2023, Mr. Adams shared a stage with Mr. Modi at the U.N.’s International Yoga Day.

Satya Dosapati, the founder of the Indian Americans for Cuomo PAC, which paid for the aerial banner over the Statue of Liberty, previously led protests against the University of Pennsylvania when Mr. Modi was dropped as a speaker there in 2013.

The Hindu American Foundation, whose members maintain relationships with the Indian government and which is the largest Hindu advocacy organization in the United States, was co-founded by Mihir Meghani, who wrote a Hindu nationalist essay adopted by the B.J.P.

The foundation has not taken a public position on Mr. Mamdani’s campaign, but its director, Suhag Shukla, attacked Mr. Mamdani in an online post as “an entitled, dilettante” and said in a statement that he has used “demonizing rhetoric” against Hindus. Other former and current leaders of the foundation have also criticized the candidate online.

Mr. Mamdani, now running in a general election and seeking a broader base of support, said recently that he would “discourage” using the controversial phrase “globalize the intifada,” which inspired the banner over the Statue of Liberty. Many see the statement as a call for violence.

The question remains whether Mr. Mamdani will similarly temper his criticism of Mr. Modi as November’s vote approaches. He declined a request to be interviewed for this article.

Since the clip of Mr. Mamdani calling the prime minister a war criminal went viral among Hindu voters, Mr. Mamdani has been less vocal about Indian politics.