A handful of prominent San Francisco political operatives are seeking to gut a ballot measure voters overwhelmingly passed last year that pulls back the curtain on who’s paying for campaign advertisements.

In a federal lawsuit filed last week, a group sued the city to neuter the key provisions of Proposition F, which its supporters named the “Sunlight on Dark Money Initiative.”

Prop. F, which passed with 77% of the vote and took effect in December, forces political campaigns to disclose their top three contributors of $5,000 or more in their advertising materials, along with exactly how much they gave. That threshold was previously $10,000.

If one of a campaign’s top three contributors is another political group — a common technique that can be used to obscure the sources of donations — the advertisements must also disclose that umbrella group’s top two contributors and the size of their donations. Among other provisions, the measure placed strict limits on contributions made by people or companies that have major real estate matters — worth $5 million or more — before the city.

The group that filed the lawsuit contends that so much mandated financial disclosure will crowd out the political messages campaigns are trying to convey through television, radio, print and online ads. The group argues that the constraints placed on political ads by Prop. F violate their free speech rights.

“The additional disclosure requirements strike me as being illegal,” said Todd David, the group’s principal officer and executive director of the Housing Action Coalition, an organization closely aligned with the mayor’s office. “I’m very concerned that this limits the ability of campaigns, particularly small campaigns to communicate.”

A twist in the lawsuit is that the group suing, led by David, was ostensibly formed to support and raise money for the upcoming earthquake safety bond on the March ballot. Another political committee, run by the San Francisco firefighters’ union, is the primary funnel for money aimed at supporting the earthquake measure and appears to be complying with Prop. F’s mandates.

The lawsuit has drawn support from two additional veteran campaign consultants with deep roots in San Francisco and Bay Area politics: Maggie Muir and Nicole Derse. Muir is a political strategist and campaign manager for Mayor London Breed, among other candidates and causes. Derse, the principal and cofounder of 50 + 1 Strategies, has run many local campaigns, most recently for former district attorney candidate Suzy Loftus.

Muir and Derse are not plaintiffs in the suit, but filed declarations with the U.S. District Court for the Northern District of California supporting a motion to block the city from enforcing Prop. F.

“Proposition F makes it more difficult for voters to hear a campaign’s message, from any side,” Derse said in a statement. “Online ads are a cost-effective way for even the lowest budget campaigns to reach voters; as the law stands now, their entire 15-second ad could be a disclaimer. That’s just not fair to either campaigns or voters.”

Muir said in a statement: “The disclosure of campaign funders is critical to ensuring transparency for voters and the reason why San Francisco has robust disclaimer rules in place. The expanded disclaimers required under Prop. F renders useless many communication tools, particularly those used by smaller, less well-funded campaigns. In a 15-second digital ad, the spoken and written disclaimers will cover the entire ad. In smaller newspaper ads, the disclaimer will fill the space of the entire ad.”

Jon Golinger, who co-authored Prop. F, called the suit an attempt by business-friendly political consultants to obscure who’s behind the donations flowing to the candidates and causes they work for.

“This lawsuit is designed to keep voters in the dark so that secretive Super PACs can hide the real agenda behind their campaign ads,” he said in a statement.

In a brief, the group bringing the lawsuit argues that the disclosures mandated by Prop. F had forced the committee to make tough choices regarding from whom it seeks funding. The group, for instance, decided not to seek money from the San Francisco Association of Realtors because it’s “commonly associated with pro-development and pro-business interests,” which could turn off voters. But David’s group has taken money from San Francisco’s United Democratic Club, which itself has taken contributions from the Realtors Association. That means the association would still have to be identified as a donor for the earthquake measure.

“Wanting to hide unpopular donors is not a valid reason to deprive voters of information. We hope that the city attorney vigorously defends this law and the will of the voters in court,” Golinger said.

John Coté, a spokesman for the city attorney’s office, confirmed the city attorney is “defending this ballot measure consistent with First Amendment law.”

Dominic Fracassa is a San Francisco Chronicle staff writer. Email: dfracassa@sfchronicle.com Twitter: @dominicfracassa