Quentin Kopp resigned from the San Francisco Ethics Commission on Tuesday because of what he called “unsatisfactory” investigative practices and a lackadaisical nature in calling out corruption in city governance.

“It is not very effective,” Kopp, 90, said of the commission, on which he has served since 2017. “I don’t think anyone is afraid of the Ethics Commission who is in competitive political life in San Francisco. The lobbyists have acclimated themselves to it over a period of 25 years. And so have the candidates.”

The commission was established by a 1993 ballot measure to enforce the city’s ethics laws and rules, which cover campaign spending and contributions, conflicts of interest and policy issues. The agency has long been criticized for lax policing of ethics rules, campaign regulations and for a growing backlog of cases.

Kopp, a former San Francisco supervisor, state senator and retired San Mateo County Superior Court judge who is known for his gruff manner, ran unsuccessfully for mayor against Dianne Feinstein in 1979.

The Board of Supervisors appointed him to the Ethics Commission in February 2017. His abrupt resignation ends his tenure four years early and leaves a vacancy for the board to fill.

“I find the investigative practices of the Ethics Commission staff unsatisfactory,” Kopp wrote in his resignation letter. “I have lost confidence in the ability of the Ethics Commission and its staff to achieve the purposes represented to voters and residents two decades ago for its establishment.”

Kopp said there is a massive backlog of cases at the commission, which he said is “ineffective and a waste of taxpayer money.”

He pointed to one case in 2017 when the commission negotiated a $25,000 settlement with former Supervisor Mark Farrell for violations related to his 2010 campaign for supervisor. The commission initially sought a $191,000 fine against Farrell, but the City Attorney’s Office advised the commission to settle the case because the statute of limitations had run out.

“That’s emblematic of what occurs when you don’t process (cases) rapidly,” Kopp said Tuesday.

The commission currently has 88 open complaints — 25 more than it had this time last year, according to commission data. The average age of cases has also increased over the past year, from about 10 months to 15.

Ethics Executive Director LeeAnn Pelham said the agency is currently short one full-time investigator and has only three staffers working through its caseload. She said complaints tend to spike during election seasons.

“That’s a good sign,” she said. “If we’re receiving complaints, people think we are going to take them seriously. ... We are doing what we can with the staffing resources we have and the complexity of the cases.”

Kopp’s resignation comes a year after another former commissioner, Peter Keane, resigned after the panel failed to bring a campaign reform measure to the June ballot.

Ethics Commission chair Daina Chiu said in a statement she was “saddened and disappointed” to learn of Kopp’s resignation.

“I am proud of the great work the commission and staff have done and are continuing to do to deepen the effectiveness of our reform laws in the city,” she wrote. “The commission’s mandate is vital and demands a deliberative body that represents a variety of viewpoints.”

Kopp said he will now spend his free time working for a November ballot measure that would reform the city’s campaign finance laws.

The measure, which was conceived by a former ethics commissioner, would require a campaign to prominently disclose the name and dollar amount contributed by its top five donors of at least $5,000 on campaign ads.

If it passes in November, the measure would also limit campaign contributions from a person or company who would have land-use decisions going before the city within 12 months. It would also prohibit limited liability corporations and limited partnerships from donating to campaigns.

“I will engage with it with full gusto in the city,” Kopp said.

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