SAN FRANCISCO — This famously tolerant city has been forgiving of its politicians’ foibles.

San Franciscans returned John Burton to power in the Legislature after a cocaine addiction drove him from Congress. They re-elected Gavin Newsom as mayor after he admitted to sleeping with his campaign manager’s wife.

Now, the San Francisco Bay Area’s fabled mercy is being put to the test in a spree of Democratic infighting, one whose angry recriminations are causing fissures throughout the state’s political establishment.

State Sen. Carole Migden — whose substantial legislative accomplishments have been overshadowed by her transgressions of driving rules, Capitol mores and campaign finance laws — is in danger of becoming the first state legislator in 12 years to be unseated by a member of her own party.

The contested primary in the 3rd Senate district — to be decided June 3 — began more than a year ago when Assemblyman Mark Leno, a fellow San Franciscan, jumped in. Viewed from afar, Leno is nearly identical to Migden in voting record (very liberal), ethnicity (Jewish) and devotion to gay rights (he is gay, she is lesbian, and both have been prime movers of major legislation).

Different roots

They diverge in style, which can be traced to their origins: Migden grew up in Yonkers, a gritty city just outside New York; Leno was raised in Milwaukee in the self-effacing Midwest.

Leno’s campaign has been predicated on the promise that he can do the job as well as Migden but without her extracurricular dramas. But the primary has exacerbated the already strained relations between Senate and Assembly Democrats by breaking an unspoken Sacramento taboo: A sitting state legislator should not attempt to unseat an incumbent from the same party.

The race’s origin is partly rooted in an old feud: The two were collegial until Migden endorsed an ally over Leno in the 2002 Assembly race he ultimately won.

Leno’s bid to oust Migden is the latest bout of musical chairs prompted by California’s strict term limits law. Leno must leave the Assembly this year, and there’s no other appealing public office that is vacant.

As if their bitter rivalry were not enough, the race has attracted another formidable candidate — Joe Nation, an economics professor and consultant who represented nearby Marin in the Assembly until term limits expelled him in 2006.

Already, business groups, including ones that want the state to restrict lawsuits against businesses and doctors, have spent $208,611 on Nation’s behalf. A coalition of horse-racing interests has spent $38,059 for Migden, and an independent committee partially funded by some of Leno’s Assembly colleagues has spent $50,000 for him.

All told, the primary “makes ‘American Gladiator’ look like sandbox play,” said U.S. Rep. Jackie Speier, D-Calif., describing the ferocity of the race.

Given the lopsided Democratic electorate in the district, which encompasses Marin, northeastern San Francisco and southern Sonoma, the winner of the primary is virtually assured election in November.