San Francisco’s narrowing middle class, already squeezed by the high cost of living could take another hit if a new business tax is approved in the November election, according to the city’s Office of Economic and Workforce Development.

The measure, approved for November’s ballot last month, would levy an average of about 0.5 percent gross receipts tax on companies that make $50 million or more in revenue. In return, it would roughly double the city’s funding for housing and homeless services, such as 1,000 extra shelter beds and services for the mentally ill.

Homelessness activists, nonprofits and community groups strongly support the measure, saying it’s time for the city’s businesses to do more to alleviate San Francisco’s homeless population.

But, according to a memo sent to the mayor’s office by the Office of Economic and Workforce Development last week and obtained by The Chronicle, the extra tax would disproportionately impact employees in mid-level jobs, such as administrative staff in retail companies and grocery store workers.

“There is a limit on how high taxes can go before you decide to go to Oakland, where the taxes are much, much lower,” Jim Lazarus, senior vice president of public policy for the San Francisco Chamber of Commerce, said of businesses located in the city. “There are companies in San Francisco that pay more San Francisco tax than they pay in other states.”

“Many of the firms facing large tax increases have significant regional and national footprints, and large shares of middle income, non-technical support and administrative roles in San Francisco — the jobs largely considered the easiest to move,” the memo said. Those are the kinds of jobs that are more easily moved to locales with lower taxes, it stated.

“We need to be conscious of who our revenue generating measures are affecting. In this case, it seems the potential to impact middle income jobs should cause concerns,” said Joaquín Torres, director of the Office of Economic and Workforce Development.

The office estimates that the revenues from the tax would come from less than 5 percent of the city’s businesses. And if passed, the measure may more than double the current gross receipts tax, costing San Francisco businesses about $280 million to $300 million a year, the memo said.

Lazarus said some businesses impacted by the measure could be grocery stores, such as Safeway and Whole Foods — as well as large retailers that are already struggling to stay afloat, such as Macy’s and Nordstrom. The memo estimated that less than 5 percent of the city’s 88,000 businesses would be subject to the new tax and that most of them would be involved in professional services, financial services, information and retail. Others could be companies that have a large administrative presence in the city, like Visa and McKesson.

Another proposal headed for the November ballot — a gross receipts tax on cannabis operations — could also add to companies’ tax burdens and would make San Francisco a more expensive place to do business.

Concerned about how this measure will squeeze out middle-class jobs from the city, the Chamber of Commerce established a coalition last week — Right Priority, Wrong Approach — to defeat the measure. It hired BMWL & Partners and 50+1 Strategies as political consultants for this campaign.

But those who support the measure, such as Evan Owski, a software engineer at LinkedIn and a large donor to the ballot initiative’s campaign, said the notion that imposing this gross receipts tax would cause large multinational companies to pull out of the city is “laughable.”

“This is a classic tactic of big business,” he said. “They keep saying ‘job killing’ over and over, which scares people, so they don’t have to keep paying for these vital social services.”

Supervisor Catherine Stefani said she is not convinced the measure would solve the city’s homeless problem.

“San Francisco already is spending more than $300 million a year on homelessness and the problem seems to be getting worse,” she said in a statement. “I’d have to be convinced that taxing San Francisco hundreds of millions more is a good idea, considering the measure contains no plan, no accountability and no reforms, and the city’s own analysis says it’s going to cost middle-class jobs.”

Mayor London Breed is assessing the potential impact of this tax “on the city’s long-term economic health,” according to a statement from her press office.

The measure exceeded the amount of necessary signatures to appear on the ballot. A poll of 400 likely voters conducted last month by David Binder Research and commissioned by South of Market affordable housing manager Todco found 66 percent in favor and 28 percent opposed to the measure.

Another poll dated Monday, conducted by EMC Research, showed less support, with 53 percent of likely voters backing the tax and 45 percent opposed. The report said that support for the tax to fund homeless services hovers above a majority, “but is vulnerable to a well-funded opposition campaign.”

“We have people power, and we can use that people power to defeat the millions that they will throw at this,” Owski said of the opposition campaign.

Christin Evans, one of the original petitioners of the ballot measure and an owner of the Booksmith on Haight Street, said by taxing companies with more than $50 million in revenue, the tax would impact companies that can shoulder the extra cost. And, she added, increasing homelessness services would be a net positive for businesses, as it will help attract workers and tourists to the city.

“These are not mom-and-pop retailers,” she said. “I think the city is at the point and is ready. What we’ve found in our drive to collect signatures (is) hundreds of people who say this is a fair and measured way of tackling the homeless crisis.”

But Alan Auerbach, a UC Berkeley professor and director of its tax policy center, said given how high sales taxes are in the city, 0.5 percent in gross receipts “isn’t negligible.”

“It will have cascading effects,” he said. “It is half a percent, it’s not 10 percent, so you might think it’s small — but unlike a sales tax, it is hitting everything.”

Gross receipts taxes are taxes on the total gross revenue of a company, regardless of the source of the revenue. Auerbach said, ideally, the city should not have to deal with funding homeless services on its own.

“Homelessness is less of a city problem than a state or national problem,” he said. “If we had any sense, we would be dealing with it at a higher level than the city. Homelessness may be in San Francisco, but it’s not just a problem that San Francisco should be controlling on its own.”

Jennifer Friedenbach, executive director of the Coalition on Homelessness and one of the measure’s backers, said she is not worried by the opposition campaign. She said the initiative has had a “ground swelling of response.”

“This issue effects all of San Francisco, and this is a really carefully crafted way to address it,” she said. “It is time to turn this around in a big way, and it will cost money to do it — and this is a fair way to get us there.”

Trisha Thadani is a San Francisco Chronicle staff writer. Email: tthadani@sfchronicle.com Twitter: @TrishaThadani