In an interview, Mr. Benioff said that homelessness had become a crisis in San Francisco and that Proposition C, which would nearly double the city’s budget for addressing the issue, was a necessary step. But Mr. Dorsey argued that the tax would unfairly affect payment companies like Square. He said the city needed to give its newly elected mayor, London Breed, a chance to deliver on her campaign promises to quell homelessness.

Jess Montejano, a spokesman for the No on C campaign, said tech companies “understand that more money alone is not the answer to our city’s most pressing issue.”

A number of local politicians, including Ms. Breed, have publicly opposed the measure, although polling conducted by the No campaign in early September indicated voters favored Proposition C by a slim margin. If passed on Nov. 6, opponents say it would be the largest tax increase in the city’s history.

“We need to invest more in homelessness, and we may need to raise taxes to do that, but this is not the right way to do it,” said Scott Wiener, a California state senator who opposes the measure. “Instead of scapegoating one industry, we need to look in the mirror and say, ‘What did we do to get here, and how do we make it better?’”

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