As spending on San Francisco’s elections reaches staggering heights, supporters of a November ballot measure want voters to have a better idea of who’s funding the campaign ads that inundate their mailboxes, TVs and social media feeds.

Proposition F — pushed by a former ethics commissioner and Supervisor Gordon Mar — would force campaigns to more prominently disclose who donated large sums of money to a cause. It would also limit contributions from those who have a land-use decision going before the city. The point? Eliminating “pay-to-play politics” that influence decisions in City Hall, said Jon Golinger, a co-author of the measure.

San Francisco’s campaign finance laws are so opaque and rife with loopholes, he said, that voters are unable to make fully informed choices at the ballot box. At issue, he said, is the ability for independent political action committees to raise an unlimited amount of money from corporations, unions and individuals. Those committees can then donate to individual campaigns, and therefore shroud the real source of the money.

“It’s long overdue. It’s just a data point to give voters more information,” he said. “They may not care, but I think it will ... allow them to ask more questions about the ads they see, rather than just taking them at face value.”

The measure — officially called Sunlight on Dark Money — comes as the amount of cash spent on local elections keeps going up and up. Last year’s local elections were the priciest in recent City Hall history, with nearly $7 million spent on the Board of Supervisors races and more than $8 million spent during the special election to replace Mayor Ed Lee after he died in 2017.

Millions of dollars have also been spent on the upcoming November election. Mayor London Breed — who faces no serious competition for her seat — has raised nearly $600,000. Vaping company Juul spent over $18 million on one measure before it pulled its support last month. Meanwhile, the two major candidates for the sole open seat on the Board of Supervisors have raised nearly half a million dollars each. The four major candidates for district attorney have each attracted heaps of cash, too.

In contrast, the campaign for Prop. F has raised $23,000.

If Prop. F passes in November, which requires a simple majority, campaign ads will have to include both the name of and amount contributed by the top three donors of $5,000 or more. If any of the top three donors are a committee, then the disclaimer must also have the name and the dollar amount contributed by each of the top two major contributors of $5,000 or more to that committee.

It would also limit campaign contributions from a person or company who has land-use decisions before the city within 12 months before or after the donation. While supporters of the measure say it is an important provision to keep politics out of the city’s housing and development decision — one of the most contentious issues in San Francisco — critics say it is unfair to specifically target developers.

The measure would also prohibit limited liability corporations and limited partnerships from donating to candidate committees.

John Dennis, chairman of the San Francisco Republican Party — the only registered opposition to the measure — worries that prominently displaying the names of contributors may have a “chilling effect” on participation in local politics. He said society has a “problem” with shunning people who have different points of view.

“We need to protect other points of view, and money represents points of view,” Dennis said. “We would like people to stay in the dark in this regard.”

Mar said Prop. F is a relatively modest measure that he hopes will spur more campaign finance reforms down the line.

“I’d like to consider additional measures that would prevent outside special interest groups from unduly influencing our election process,” he said. Mar added that he is not exactly sure what he will pursue next, but he’s “in discussions with a range of stakeholders.”

The measure also comes on the heels of a number of other changes to the way candidates receive money during campaigns. The Board of Supervisors passed two ordinances earlier this year that change the city’s public financing program. But like those changes, the provisions in Prop. F do not need to go through the ballot and instead could be figured out through the legislative process.

Golinger said he didn’t trust that City Hall would make the changes on its own in time for the next election, and felt like the voters should have a say in the matter.

“These problems haven’t been solved in the past by the board or the Ethics Commission,” he said. “Going to the voters makes sure that it gets done.”

If passed, City Controller Ben Rosenfield said the San Francisco Ethics Department would incur some additional staff costs related to monitoring and enforcing the changes. He estimates that one-time costs for software development of new reporting requirements would be $50,000 to $100,000.

The measure was written by former San Francisco Ethics Commission Chairman Peter Keane. Four other former ethics commissioners endorsed it, and eight members of the Board of Supervisors co-sponsored it: Supervisors Sandra Lee Fewer, Aaron Peskin, Vallie Brown, Matt Haney, Norman Yee, Rafael Mandelman, Hillary Ronen and Shamann Walton.

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