Richard David’s face is plastered around the Richmond Hill neighborhood of Queens. Along Liberty Avenue, on posters in the windows of sari shops and roti restaurants, Mr. David advertises his campaign for City Council, which, if successful, would make him New York’s first city councilman of Guyanese descent.

One of Mr. David’s campaign promises: securing money for resources like immigration lawyers or language assistance for the diverse neighborhood that includes Little Guyana, a hub of the largest Guyanese community outside of the country itself.

It is also the community in New York City that could lose the most from a new federal effort to cut legal immigration in half, in part by limiting what are known as family preference visas, which go to the siblings, grandchildren, in-laws or adult children of United States citizens, as well as the spouses and children of legal permanent residents. That is exactly the kind of visa that allowed Mr. David to immigrate from Guyana in 1995, and that helped bring other members of his family into the country, too, as recently as last month.

“Eight family members of mine just came through family sponsorship on the Fourth of July,” Mr. David said. His grandmother sponsored two adult daughters, who also brought their children. Of the proposed immigration bill, which was endorsed by President Trump last week, Mr. David said, “This could cease or significantly reduce Guyanese migration to the country.”

It is unclear if the bill will ever become law.

The Guyanese community brings in more people through family preference visas than any other immigrant group in the city. Of the Guyanese in New York City who received legal permanent residence between 2002 and 2011, 60 percent entered on family preference visas, according to a 2013 report by the Department of City Planning. Thirty-seven percent entered as immediate relatives, an uncapped visa category that includes the spouses, parents and minor children of citizens.

Foreign-born Guyanese people make up a tiny share of the United States as a whole — just over 280,000 people in 2015, or 0.09 percent of the total population — but a hefty share of New York City’s immigrant population. More than half of the Guyanese population in the United States lives in New York City, according to city data, making it the fifth-largest immigrant population in the five boroughs and the second-largest in Queens.

“Their propensity to come to New York City is very high,” said Joseph J. Salvo, chief demographer at the Department of City Planning. “And they are heavily reliant on family preferences — and reliant on categories that, under this proposal, would disappear. There’s no question that they would be affected in a dramatic fashion.”

The bill, sponsored by Senators Tom Cotton of Arkansas and David Perdue of Georgia, seeks to reduce the number of people granted legal permanent residency each year — currently more than one million — by 41 percent in its first year and 50 percent by its 10th year, according to its sponsors’ estimates.

To do that, it proposes narrowing the definition of immediate relatives, removing parents from the list and lowering the age of qualifying children to 18 from 21. Siblings of citizens, as well as the adult children of citizens or permanent residents, would no longer be eligible for family sponsorship. The total number of family preference visas would be cut to 88,000 a year, a 60 percent reduction from the current 226,000.

Of New York City’s Guyanese immigrants who became legal permanent residents from 2002 to 2011, 45 percent were the parents, married children or siblings of citizens, or their spouses or children, according to the city’s data. If the proposed bill had been law at the time, nearly half of new Guyanese immigrants to the city would have been ineligible.

For a community that relies upon tightly knit family units, where multiple generations live together in one house and grandparents often care for grandchildren while parents work, the constriction of family immigration would be especially wrenching.

“In our Guyanese community, nuclear family is not tied down to mother, father, children,” said Deborah Assanah, 56, associate director of the Guyana Cultural Association. “We have like a village of family members who assist with raising the kids.”

The Guyanese community, which includes people of Indian, African, Chinese and indigenous descent, has one of the highest rates of female labor force participation among New York City immigrants, perhaps aided by the availability of extended family to care for young children, said Philip Kasinitz, a sociology professor at the City University of New York.

And because many Guyanese immigrants send remittances to relatives at home, cutting off family immigration would effectively make immigrants responsible for financially maintaining two households, with no prospect of reunification, said Vishnu Mahadeo, president of the Richmond Hill Economic Development Council.

Additionally, many Guyanese parents prefer for their children to come to the United States either as very young children or after they have completed their education, so that they can integrate more easily into American society or the work force, Ms. Assanah said. But that means many are older when they immigrate, making them targets of the new proposal. Ms. Assanah immigrated in 2008, sponsored by her husband, who is a citizen. A few years earlier, he also sponsored their twin daughters, who were 21 at the time — older than the proposed new cutoff.

Many Guyanese had not yet focused on the bill. Vrinda Jagan, a lawyer in Richmond Hill who works on immigration, said that immediately after the November election, clients flooded her office with questions and pleas for reassurance that they would not be deported en masse. But nobody had asked her about the new bill, she said. And applications for family sponsorship have not flagged since President Trump’s endorsement of it.

“I sent out a few this week,” she said. “They’re continuing to petition for their family members, and they’re petitioning for their spouses, their children, a lot of children over 21. That hasn’t changed.”

The policy would be most devastating to people whose applications for family-sponsored visas had been pending for years, said Randy Capps, director of United States research at the Migration Policy Institute.

Especially for those with lower priority cases, like siblings or married children, the wait can be 10 years or even more.

At Singh’s Roti Shop and Bar on Liberty Avenue, Sandra, who asked to be identified by only her first name because she feared disrupting her immigration proceedings, said she had been waiting on family sponsorship for 12 years; she was in the United States on a tourist visa.

She was being sponsored by her sister-in-law, who had submitted an application for her brother, Sandra’s husband. Under the current system, Sandra and her three children would be allowed to enter as well.

If the bill were passed, Sandra said, “I would feel bad, because you wait so long.” Her children, who have never been to the United States, are eagerly awaiting the day the application is approved. “They’re so excited,” she said.