Just six days into Donald Trump’s presidency, hundreds of volunteers and officials will fan out across San Francisco to take a count of the city’s homeless population, hoping to get the most accurate tally possible — and qualify the city for the most federal money possible.

The key word here is “possible.” When it comes to the count, done every two years, accuracy is far from a given. And it’s hard to know how much any degree of accuracy is going to matter with the new administration and Congress.

As a candidate, Trump promised to slash discretionary social service spending. Republican budget proposals take that goal even further, and the man nominated by the new president to head the federal housing and homeless funding agency is viewed with skepticism by many antipoverty activists.

Retired neurosurgeon and former GOP presidential candidate Ben Carson, tapped by Trump to lead the Department of Housing and Urban Development, has no experience in government and has been quoted as saying that “poverty is really more of a choice than anything else.” House Minority Leader Nancy Pelosi, D-San Francisco, called Carson “disturbingly unqualified” for the post.

During his confirmation hearing this month, Carson struck a moderate tone, reminding senators that he had grown up “close to being homeless.” He said that although he intended to evaluate HUD programs with an eye toward making reductions, he thought some were effective, particularly housing vouchers for homeless veterans.

“Although homelessness is down, even among veterans, we must continue to tackle this problem by continuing to build strong partnerships with counties and cities across America,” Carson told the Senate Banking, Housing and Urban Affairs Committee. “I want to build on this progress.”

The statements did little to reassure many advocates for homeless people.

The Trump administration is “going to be a disaster,” said Paul Boden, organizing director of the Western Regional Advocacy Project in San Francisco, which works on behalf of poor people.

He said the federal government has never fully restored the money cut from antipoverty and housing-aid programs under President Ronald Reagan in the 1980s. In current dollars, funding levels are less than half what they were 35 years ago, according to studies by Boden’s organization and others, including Georgetown University.

Boden, whose activism dates to Reagan’s time, sees no improvement in sight.

“A guy who’s a brain surgeon ... will now be in charge of housing,” Boden said. “That pretty much tells you everything you need to know. It’s all going to get worse.”

Former San Francisco Mayor Art Agnos, who served as Western regional head of HUD from 1993 to 2001, said that, despite Carson’s statements to the Senate committee, the nominee “is ignorant and uninformed about homeless and housing issues ... and, quite frankly, in the HUD bureaucracy there is no strong inclination toward dealing with homelessness, either. If anything, services will diminish. At best they will stay the same.

“The advocacy for homeless funding always came from political appointees” to HUD, Agnos said. “I hope as Carson gets informed, he will understand that people sometimes need a hand. But if he sticks to his ideology that everyone has to pull themselves up by their bootstraps, then we’re not going to get much help.”

A rare cautious-but-optimistic note came from Sherilyn Adams, director of Larkin Street Youth Services, which helps homeless young people — who made up 21 percent of San Francisco’s street population in the 2015 homeless census. She is on the executive committee of A Way Home America, a national consortium of advocates and agencies that is crafting a transition plan for the new president to end youth homelessness by 2020.

“There are obviously concerns about the placement of Trump’s Cabinet members, but we can’t let that define our work or what we need,” Adams said. “It only informs how we have to advocate for the system’s change and resources. I am a born optimist.”

San Francisco spends $265 million a year on homeless services and supportive housing for formerly homeless people, and as much as half of that is supplied by the federal government.

The funding from Washington includes the McKinney-Vento homeless grant, disbursed annually by HUD to cities throughout the nation. This month, San Francisco learned its allocation for the coming year is $32 million, up from $27 million in 2015 when the last homeless count was taken.

Millions of additional dollars arrive through Medicaid, housing vouchers, disability payments and dozens of other federal programs for the poor. With Trump promising on the campaign trail to cut 1 percent of all discretionary nondefense spending every year for a decade, and congressional Republicans floating plans to cut up to $3 trillion from low- and moderate-income programs during the same period, the ripple effect could turn into a tsunami.

“The private market just won’t provide housing for poor people without government subsidies, and there is real risk here that those subsidies could take some cuts,” said Sarah Edleman, director of housing policy at the Center for American Progress, which studies poverty. “In the previous session of Congress, Republicans tried to gut this kind of funding, but HUD had a strong leader who stood up to them. Now there is real risk.”

Whether the funding goes up or down, San Francisco gets none of that core McKinney grant without its biennial street count. And although the city’s federal funding is partly dependent on the one-night census — the more people, the better the chances of more money — officials concede that the count far underestimates the true number of homeless people.

For more than a decade, cities and counties have been estimating the number of homeless people the same way — by sending volunteers and government officials out for one night to streets, shelters and anywhere else not considered a permanent home to do a visual count. However, they don’t stop to interview anyone, and there are plenty of places where sleeping people can’t be seen.

The count always takes place during January, because the government assumes more people will be in shelters during the middle of winter and thus available for head counts. San Francisco will do its count Thursday night.

The city’s assessment in 2015 came up with 6,686 homeless people, an increase of 3.8 percent from two years earlier.

The San Francisco Coalition on Homelessness estimates a more realistic count would be 13,000 if those who were hiding, sleeping on a friend’s couch or otherwise not readily visible were tallied. The city Department of Public Health recorded 9,975 homeless people who used its services in fiscal 2014-15.

“These asinine counts are completely inaccurate, and it’s time we focused on more dollars for housing instead,” Boden said.

Jeff Kositsky, whom Mayor Ed Lee named last year to head the new city Department of Homelessness and Supportive Housing, said that as flawed as it is, the count “is still important because it provides us an apple-to-apple comparison to other counties and locally over the years.”

Kositsky anticipates San Francisco’s numbers could go up a bit again — not so much because significantly more people are on the streets, but because they are being pushed into more visible spots as developers bulldoze their hiding places. He also anticipates “an uptick in counts all along the West Coast, because this is where the national economy is and poor people go where the opportunity is.”

In 2015, homeless counts elsewhere along the West Coast rose more than in San Francisco. King County in Washington, where Seattle is, saw an increase of 10 percent. Los Angeles County’s street count went up 12 percent.

HUD also reported that from 2010 to 2016, the January counts found that the number of chronically homeless people in San Francisco went down 44 percent, better than the 27 percent drop nationwide.

Kositsky declined to predict what to expect from the new administration when it comes to funding, saying he’s concentrating on putting a system in place that will help his city department track its contacts with homeless people. That system will be coming online regardless of how much aid Washington provides, he said.

Having a real-time database, Kositsky said, will allow him to target services by determining exactly how many homeless people need drug rehabilitation, subsidized housing or specialized health care. No matter who is running Washington, that will make spending more efficient, he said.

“A simple point-in-time count is not that useful as a planning tool,” Kositsky said. “What I’m really looking forward to is getting our new data system up and running.”

Kevin Fagan is a San Francisco Chronicle staff writer. Email: kfagan@sfchronicle.com

S.F. homeless count

Officially called the “S.F. Point In Time Homeless Count,” the census is conducted once every two years, in January. The next one will take place Thursday from 8 p.m. to midnight.

A record 600-plus volunteers have signed up to help, and registration is now closed.

Volunteers and officials conduct the count in every district of the city. Results are expected to be released in the spring.