Larsen gave $500,000 on Sept. 5 to a political action committee backing Ellington, then contributed another $40,000 on Sept. 29. He said the additional money would cover expenses for the team managing the PAC. Ellington has said the contribution came from a third party he does not control and called its fallout “more trouble than it’s worth.”

The donation has become a central campaign issue in a district that includes Bayview-Hunters Point, Visitacion Valley and Potrero Hill. Brookter, a former chief executive of Young Community Developers, and J.R. Eppler, a small-business attorney, have joined in an “anti-billionaire” effort. They are asking supporters to rank one of them first and the other second on the ranked-choice ballot.

The alliance’s slogan, “District 10 can’t be bought,” reflects concerns that wealthy outside donors could influence decisions in communities facing economic pressure and the prospect of major change. Ellington has said residents will not be forgotten or forced out, and that the contribution will not determine his policy positions.

Ellington entered the final stretch with the strongest financial and endorsement backing among the three candidates. He has received nearly $488,000 in direct donations and public financing, as well as the endorsement of Mayor Daniel Lurie. The PAC spending gives his supporters additional resources for campaign operations, including door-to-door canvassing.

The joint strategy could help Brookter, who has a larger progressive following and the endorsement of termed-out Supervisor Shamann Walton, though he has raised less than Eppler and substantially less than Ellington. Former Mayor Art Agnos has backed the coordinated ranking effort. San Francisco has seen similar alliances among candidates using ranked-choice voting, but political consultant Todd David, an Ellington donor, said such efforts have often failed. David said the large contribution helps Ellington if it is spent effectively. Former Mayor Willie Brown, who has endorsed both Ellington and Brookter, said Ellington would have “fewer things to explain” without the donation. Former Supervisor Aaron Peskin, a Brookter supporter, argued the contribution could burden Ellington politically even if he wins.

The spending has also renewed scrutiny of Ellington’s past openness to Flock surveillance technology. Larsen is a leading donor to the technology in San Francisco and helped establish the Police Department’s Real-Time Investigation Center, which uses Flock cameras and drones. Data from Flock cameras has been shared with immigration agents, and the system has prompted privacy concerns. Ellington has said he would not vote to renew the city’s Flock contract without strict limits on how long data is retained.

The contest is unfolding amid broader concerns about development and displacement. Alexander Lurie, the mayor’s brother and a real estate investor, recently described Bayview-Hunters Point as a potential “sleeper” investment opportunity. Ellington said he understands residents’ concerns, adding that he was born in the district and lives there.

Even as the campaign debate focuses on large donations and development, the candidates say residents often ask about basic services: safer streets, transit, speed bumps and whether their supervisor will answer the phone. Many voters remain undecided, and turnout is a significant uncertainty. In 2018, when Walton first won the seat, turnout was 58% among registered District 10 voters, compared with 78% citywide. Turnout was particularly low in Bayview-Hunters Point and Visitacion Valley.

Campaigns are also seeking to reach Chinese-speaking voters, who account for about 42% of the district’s voting-age population, according to figures cited in the race. Ellington says his campaign is increasing outreach to voters who speak only Chinese. Brookter’s campaign has recruited volunteers for phone banking and door-knocking in Cantonese and Spanish.

Agnos said he could not predict the outcome, but argued that voters would object to the scale of the outside spending if they knew about it. Whether that message resonates may depend in part on how many residents hear it—and turn out to vote.