It's not exactly an October surprise, but it is an escalation of the ongoing parade of negative attention on Mark Farrell, who by some polls has been the frontrunner in the San Francisco mayoral race.

Former mayors Willie Brown, Frank Jordan, and Art Agnos have co-signed a letter calling for the district attorney and the state's attorney general to investigate the Mark Farrell campaign, with just four weeks left before Election Day. The focus of their complaint is the accusation — first raised in August — that Farrell's mayoral campaign is illegally using money flowing into the Yes on D campaign for his campaign's expenses.

The letter, also co-signed by two former supervisors and former State Senator Mark Leno, calls attention to "a remarkable situation wherein Mark Farrell appears to be willfully violating election law," and requests a criminal inquiry by the state and city.

The letter specifically cites a mid-August Chronicle piece, by columnist Emily Hoeven, in which Hoeven pointed to evidence of potential comingling of monies donated to the Yes on D campaign and Farrell's campaign, including a $500,000 donation from real estate magnate Thomas Coates and his wife — which came in just 10 days after the PAC, Neighbors for a Better San Francisco Advocacy, returned a $500,000 donation to the Coateses after announcing it would not be launching an independent expenditure committee to back Farrell.

Mayoral campaigns have contribution limits of $500 per person, but PACs supporting candidates, and those supporting ballot measures, don't have such limits, and the implication is that the Prop D funds, and even mailers supporting Prop D, have been doubling as Farrell campaign support. Mayor London Breed publicly withdrew her support for Prop D in August, making claims that it was just a "slush fund" for her opponent, Farrell.

The letter also cites a September Mission Local piece that argued that a series of pro-Prop D mailers violate the law.

"While candidates can use ballot measure committees to pay for advertisements that have the effect of increasing their visibility with voters, those advertisements actually have to be about the ballot measure — not the candidate’s qualifications for office or public policy issues that have nothing to do with the underlying measure," the signers write.

The letter further argues that Farrell and his campaign have violated the law by employing an Arizona-based law firm to vet its activities, a firm whose principal attorneys are not licensed to practice law in California. "Merely giving legal advice can constitute the unauthorized practice of law," the letter says.

It should be noted that while this escalation could help Mayor London Breed's candidacy and may have been orchestrated by Brown for such purpose, Brown, Agnos, and Jordan do not share the same political views and have endorsed different candidates. As the Chronicle notes this morning, Agnos, who was the last progressive to hold the mayor's office, has endorsed fellow progressive Aaron Peskin; and Jordan has thrown his support behind Daniel Lurie.

The other cosigners of the letter are former SF supervisors Angela Alioto and Quentin Kopp, former City Attorney Louise Renne, and attorneys John Keker and Randy Knox.

The letter concludes by warning that if the city and state fail to act, "Mark Farrell will have exploited inaction by ethics officials and law enforcement alike to unlawfully funnel hundreds of thousands of dollars into his campaign for mayor and perhaps prevail as a result."

Farrell hasn't yet commented on the escalation, but he has previously relied on a pat answer about his attorneys having reviewed and vetted everything he's done. As KQED's Marisa Lagos shot back while moderating that final mayoral debate last month, "That doesn’t mean it’s necessarily legal though!"

Previously: Mark Farrell Failed to Disclose $675K Loan In Election Filing