The bipartisan approach that has dominated federal homelessness policy for more than two decades is under growing conservative attack.

The policy directs billions of dollars to programs that provide homeless people with permanent housing and offer — but do not require them to accept — services like treatment for mental illness or drug abuse. The approach, called Housing First, has been the subject of extensive study and expanded under presidents as different as George W. Bush and Barack Obama. President Biden’s homelessness plan makes Housing First its cornerstone and cites it a dozen times.

But Housing First has become a conservative epithet.

Republican lawmakers, backed by conservative think tanks and programs denied funding by Housing First rules, want to loosen the policy’s grip on federal dollars. While supporters say that housing people without preconditions saves lives by getting them off the streets, critics say it ignores clients’ underlying problems and want to shift funding to groups like rescue missions that demand sobriety or employment. Some even blame Housing First for the growth in homelessness.

“No more Housing First!” said Representative Andy Barr, Republican of Kentucky, after introducing a bill last month that would offer more money for programs with treatment mandates.

Senator J.D. Vance, Republican of Ohio, used two recent hearings to argue that Housing First ignores the root causes of homelessness. The Cicero Institute, a Texas policy group, is promoting model state legislation that bars Housing First programs from receiving state funds. A documentary it produced with PragerU, a conservative advocacy group, cuts between critiques of Housing First and footage of people living in tents on the street and shots of drug use.

The escalating war over an obscure social service doctrine is partly an earnest policy dispute and partly an old-fashioned rivalry between groups seeking federal funds. But it is also a new ideological and political flashpoint, with former President Donald J. Trump and others on the right using it to to promote their argument that homelessness in liberal cities is an indictment of Democratic governance more broadly.

Joe Lonsdale, the tech mogul behind the Cicero Institute, has called Housing First part of a “Marxist” attempt to blame homelessness on capitalism, and Mr. Trump, in seeking a return to office, has pledged to place homeless people in “tent cities.”

“The attack on Housing First is the most worrisome thing I’ve seen in my 30 years in this field,” said Ann Oliva, chief executive of the National Alliance to End Homelessness, an advocacy group with bipartisan roots. “When people have a safe and stable place to live, they can address other things in their lives. If critics succeed in defunding these successful programs, we’re going to see a lot more deaths on the street.”

Until Housing First emerged a generation ago, services for homeless people were built on a staircase model: Clients were meant to progress from shelters to transitional programs, where training or treatment would ready them for permanent apartments. In practice, services were weak and failure rates high, with large numbers of noncompliant people returning to the streets.

The new approach flipped the script, offering housing first — subsidized apartments with no preconditions — and hoped that residential stability would promote further advancement. Supporters emphasized that Housing First was not “housing only”: it included services like psychiatric treatment, but on a voluntary basis.

Though skeptics feared that troubled people would leave or get evicted, early results were impressive.

Citing such studies, supporters praise Housing First as unusually “evidence based.”

Contemporaneous research also offered hopes of cost savings. While most people entering shelters were quickly rehoused, work by Dennis Culhane of the University of Pennsylvania showed that a small minority became chronically homeless and consumed tens of thousands of dollars of services in jails and emergency rooms — roughly what it cost to house them. Supporters hoped Housing First would prove “not only more humane but for some people potentially cheaper,” Mr. Culhane said.

Housing First exploded from a model to a movement under a Republican administration. Philip F. Mangano, the Bush administration’s top homelessness official, proved relentless in promoting Housing First programs, and the approach, which initially targeted the chronically homeless, broadened to a wider range of people experiencing homelessness.

The Obama administration placed a preference for Housing First into the main federal grant programs, which now provide about $3 billion a year to local groups. From 2007 to 2016, chronic homelessness fell by more than a third.

For social workers used to seeing people languish on the streets, a breakthrough seemed at hand.

“I can still feel the emotion — ‘Wow, we can house everyone!’” said Adam Rocap, deputy director of Miriam’s Kitchen, a social services agency in Washington. Optimism about ending homelessness ran so high, he said, some of the agency’s staff members asked if they should seek other jobs.

Since 2007, the stock of permanent supportive housing has more than doubled to 387,000 beds, while the Department of Housing and Urban Development found 582,000 people were homeless on a single night last year, and researchers estimate the number experiencing homelessness in a year could be three times as high.

Some recent studies have noted limits on what the programs achieve. The National Academies of Sciences, Engineering, and Medicine in 2018 found “no substantial evidence” that supportive housing improved clients’ health. Likewise, the medical journal The Lancet found “no measurable effect” on the severity of psychiatric problems, addiction, or employment.

And despite hopes, the programs did not save money. Supportive housing is expensive to build (average costs in high-priced Los Angeles, which has an ambitious Housing First initiative, are nearly $600,000 per unit), and the share of unhoused people who consume costly services is low.

Still, proponents say Housing First has succeeded where it matters most — getting people off the streets.

“Getting people out of homelessness quickly is more important than anything, because life on the streets is so dangerous,” said Professor Culhane, of the University of Pennsylvania. “The evidence shows that Housing First is a very successful policy. Undoing it would be a disaster.”

The growth in homelessness and the visibility of encampments in some locations have intensified debate. Since 2015, the unsheltered population has grown by about 35 percent, with California the center of the crisis. Most analysts say soaring rents play a major role. But critics fault Housing First for financing costly permanent housing instead of shelters that could serve more people, and for preventing treatment mandates they say would promote recovery and employment.

“I thought it would help the few and leave thousands out on the streets, and my fears have been solidified,” said the Reverend Andy Bales, chief executive of the Union Rescue Mission in Los Angeles, which enforces sobriety rules and does not get federal funds.

Housing First defenders scoff at the charge that it promotes homelessness.

“Blaming Housing First for the rise in homelessness is like blaming aspirin for headaches,” said Jeff Olivet, head of the Biden administration’s Interagency Council on Homelessness.

Mr. Olivet noted that the Department of Veterans Affairs has used Housing First policies — with more generous funding — and cut veterans’ homelessness since 2010 by more than half.

“That’s a proof point for showing we can end homelessness and end it with a Housing First approach,” he said. “What we need to do is scale it up.”

Like its predecessors, the Trump administration initially embraced Housing First, with the housing secretary, Ben Carson, praising a “mountain of data showing that a Housing First approach works.”

That changed in 2019 as California’s homelessness crisis worsened and Mr. Trump began highlighting the issue to criticize the state’s “liberal establishment.”

The Council of Economic Advisers issued a report skeptical of Housing First, and the Trump administration fired its homelessness coordinator, a holdover from the Obama years. His replacement, Robert Marbut, backed strict work and sobriety rules and said he favored “Housing Fourth.”

In a recent interview, Mr. Marbut said he was brought in to “do everything we could to reverse Housing First.”

But when the Trump administration tried to delete the Housing First preference in federal grants, congressional Democrats blocked the effort. With the coronavirus pandemic consuming the rest of Mr. Trump’s term, policy remained unchanged.

Still a revolt had been seeded. Conservative literature on the topic emerged, with critiques from the Manhattan Institute, the Cicero Institute, and the Texas Public Policy Foundation, and a Heritage Foundation paper by Christopher F. Rufo, the activist who turned “critical race theory” into a war cry on the right.

Tonally, the criticisms occupy two registers. Mr. Trump has described people experiencing homelessness as “violent and dangerously deranged,” and a Cicero Institute podcast asked whether phrases like “vagrants, bums, tramps” are preferable to “homeless.” But Cicero’s film offers sympathetic portraits of recovering addicts, and a former shelter director cries onscreen as she calls Housing First “one of the most oppressive things we’ve done” to the needy.

Cicero’s work has drawn particular attention, given Mr. Lonsdale’s wealth as a co-founder of Palantir, the data-mining firm, and his support of conservative causes. The group’s model legislation restricts encampments to designated sites and blocks Housing First programs from state funds.

“As an all-encompassing model for addressing homelessness, Housing First has failed,” said Judge Glock, who until recently led the group’s work.

Texas and Georgia have adopted measures that enforce camping bans, and Missouri passed a broader Cicero-inspired bill last year, blocking Housing First programs from state funds. Its State Senate sponsor, Holly Thompson Rehder, a Republican, said concerns about the status quo had grown after an encampment fire under a Kansas City bridge killed one person and closed Interstate 70. Even in her rural district, campgrounds complained of losing business because customers feared encampments nearby.

Ms. Rehder, who experienced homelessness as a child, said Cicero recruited her in part because of that history. Having watched relatives struggle with mental illness and addiction, she considered treatment mandates “a no-brainer.” The institute organized a study tour in Texas for her, and Mr. Glock testified for the bill.

“They were incredibly helpful,” she said.

In Congress, Mr. Barr, the Kentucky Republican, got involved after shelters in his Lexington-area district complained they could not get federal funding because of sobriety rules. He said residents told him they would have relapsed in less strict environments.

But Mr. Olivet, the Biden administration official, said critics have forgotten how often services failed the homeless before Housing First came along.

“Housing First saves lives every day,” he said. “It’s a proven intervention. We need more of it.”