SAN FRANCISCO — Audrey Sylve, a retired bus driver, has spent 13 agonizing years on a waiting list for a federal voucher that would help cover rent for an apartment in one of America’s most expensive housing markets.

This summer it seemed that help was finally on the way.

In late July, congressional Democrats introduced a $322 billion plan to bolster low-income housing programs as part of the $3.5 trillion social spending plan embraced by President Biden. At its center is a $200 billion infusion of aid for the country’s poorest tenants, which would allow another 750,000 households to participate in a program that currently serves two million families.

Affordable-housing advocates saw it as a once-in-a-generation windfall that would allow local governments to move thousands of low-income tenants like Ms. Sylve, 72, off waiting lists and to expand aid to families at the highest risk of homelessness.

But optimism has given way to anxiety. Low-income housing, and the voucher program in particular, are among those most at risk of being sharply scaled back as the White House seeks to slash the package to accommodate the demands of two centrist Democrats, Senators Joe Manchin III of West Virginia and Kyrsten Sinema of Arizona, according to several people involved in the talks.

Congressional negotiators are seeking to cut the overall size of the 10-year package, in coordination with the White House, to between $1.9 trillion and $2.3 trillion. Housing is just one of several high-price priorities on the chopping block in the negotiations.

Yet proponents say no other proposal is likely to have as immediate an effect on the lives of the country’s most vulnerable as the increase in rental assistance because it addresses a foundational problem: securing an affordable place to live when rents everywhere are outpacing earnings.

“I’m all for funding early childhood education, child care and the expansion of health care with education, but we cannot be successful with any of that unless people have safe and secure housing,” said Representative Maxine Waters, a California Democrat who leads the House Financial Services Committee, which drafted the original plan.

Supporters of the expansion say every penny is required to begin addressing a crisis that threatens to undermine recent gains in the fight to reduce poverty. They fear it will be elbowed aside by other programs, such as universal child care, that enjoy broader political support because they benefit middle-class, and not just poor, people.

“Better health care or increased educational access doesn’t do much for families sleeping in their car or under a bridge, or for the millions more on the verge,” said Diane Yentel, president of the National Low Income Housing Coalition, which is pressuring the White House to fund the program as it was drafted. “There are no ‘savings’ to be had here.”

The financial services industry, which puts together the complex public-private financing packages used to build most affordable developments, has already factored in a significantly scaled-back congressional compromise.

“Much of the proposed $400 billion in housing-related grants and tax subsidies is likely to be cut from the reconciliation bill,” analysts from Goldman Sachs wrote in an email last week. That figure bundled the $332 billion package, which also includes increases for public housing authorities and an affordable housing construction fund, with smaller package of tax breaks in the bill.

White House officials say they have made no decisions. Ms Waters and her counterpart in the Senate, Sherrod Brown, a Democrat of Ohio, said they would not accept any deal that cut the housing plan more than any other proposal.

“We’re not going to scale back. We’re not going to lose our way on this,” Mr. Brown, chairman of the Banking Committee, said in an interview. “And we’re not going to compromise the mission of transforming the fight on poverty.”

Over the past two decades, the federal government has stopped bankrolling construction of government-run public housing projects. Instead, it has shifted resources to voucher programs, which bridge the financial gap between what a poor tenant can afford to pay and what a landlord might reasonably expect to get on the open market.

Demand far outstrips supply: One recent study found that the federal government has provided funding for only a quarter of the vouchers needed to help house eligible families — and many housing authorities have simply stopped taking names to avoid leaving tenants in the lurch.

Even if the voucher increase somehow makes it past Mr. Manchin and Ms. Sinema, it would represent only a down payment on an enormous unmet need for housing aid exacerbated by rocketing real estate values in most major cities.

California’s estimated share of the new aid would bankroll only about 138,000 of 1.2 million new vouchers needed to meet the demand, said Matthew Schwartz, president of the California Housing Partnership, a nonprofit that works with community groups to finance low-income housing projects.

But it would be a significant improvement, Mr. Schwartz said, particularly on top of a $22 billion affordable-housing plan that Gov. Gavin Newsom signed into law this summer.

Joseph Villarreal, executive director of the housing authority of Contra Costa County, outside San Francisco, is less concerned about the future than fulfilling the promises he has made in the past. He saw the new cash in personal terms, as a way to fulfill a commitment more than a decade in arrears.

“It would be horrible if any, much less the majority, if this voucher money gets cut from the proposal,” he said.

Mr. Villarreal’s organization, which serves as a pass-through for federal funding, maintains 51 separate waiting lists for the vouchers — some for specific developments, others for targeted demographic groups, with 47,000 families in limbo. “It weighs on me,” he said of the lists.

Ms. Sylve, who said she was scraping by on a small pension and Social Security, was one of 6,000 chosen from 40,000 qualified Contra Costa County applicants in a lottery to be added to the slow-moving queue for the program, which is still known by its historical name, Section 8.

A few years ago she was told that a voucher was about to become available, but that fell through, and she has spent much of the past 13 years hopping from apartment to apartment. Last spring, Ms. Sylve moved in with her daughter across the bay in San Francisco, because the neighborhood around her apartment had become too dangerous.

“They give you hope, and that’s the hardest part,” Ms. Sylve said. “But you keep hoping, year after year after.”

Shaunice Kyle is caught in that same limbo. She was pregnant when she went on the Contra Costa list in 2008.

“My daughter’s 12 now,” Ms. Kyle said. “Still no voucher.”

A survey of 44 large housing authorities across the country conducted by the Center on Budget and Policy Priorities, a left-leaning Washington think tank, painted a grim picture of the voucher program. A total of 737,000 people were on waiting lists, and 32 of the authorities are refusing to take new applications, with a few exceptions for particularly vulnerable populations.

The situation on the West Coast was especially dire, with eight times as many people lingering on waiting lists as receiving aid in San Diego, where the list has topped 108,000. Long waiting lists are also a staple in Washington, Philadelphia, Houston, Honolulu, Little Rock, Ark., and New York, which closed its list years ago.

Will Fischer, director for housing policy for the center, said bolstering the voucher program was the most important single move the federal government could make to address the homelessness crisis.

“Look, the public housing money is urgently needed — but it would be for existing units, for families who already have a place to live,” he said. “And most of the other funding in the proposal actually serves people a little bit higher up the income scale.”

Representative Ritchie Torres, a Bronx Democrat whose district is among the poorest in the country, said housing always seemed to be listed as the third, fourth or fifth priority of many liberal lawmakers.

When House Democrats peppered Mr. Biden with questions about the social spending package at a meeting in the Capitol this month, Mr. Torres — a former chairman of the New York City Council housing committee — was stunned when he realized no one had asked the president about rental aid, and spoke up.

Mr. Biden responded by promising he would “protect” housing, without elaborating, Mr. Torres said.