SAN RAFAEL – Assemblyman Mark Leno broke a cardinal rule in the Capitol when he decided to challenge Sen. Carole Migden, a lawmaker from his own party.

In California, state lawmakers rarely face contested re-elections, especially from a fellow Democrat or Republican. Not since 1996 has a member of the Legislature seeking re-election been defeated in a primary.

Leno's decision resulted in the Legislature's most high-profile race this year, a three-way contest in the 3rd Senate District that also features former Assemblyman Joe Nation. The district stretches from eastern San Francisco to Marin and southern Sonoma.

An elected official since 1990, Migden is battling to save her career after a year of disastrous headlines that included paying a record $350,000 fine for campaign finance violations.

She also pleaded no contest to misdemeanor reckless driving after a 30-mile ride down Interstate 80 in which she bounced off a center divider and rear-ended a car in her state-issued SUV in Fairfield.

Migden and Leno are both San Franciscans, gay and liberal. She is best known for legislation that provided benefits in California for domestic partnerships while he authored legislation, twice vetoed by Gov. Arnold Schwarzenegger, that would have recognized same-sex marriages.

But Leno has a reputation for being as congenial as Migden is brusque. Although he announced his Senate candidacy before Migden's wild ride and record fine, Leno has capitalized on her missteps.

In a new poll conducted for a Bay Area television station KPIX, Leno had the support of 42 percent of respondents, while 22 percent favored Nation and 21 percent Migden.

At a recent candidates forum in San Rafael, sponsored by the Marin County Bar Association, Leno said he decided to run for the Senate "convinced that democracy is best served when voters have a choice."

Leno is being termed out of the Assembly seat he won after Migden backed another candidate to succeed her. He told the Marin County gathering that if he hadn't decided to challenge Migden, "we wouldn't be here today, there probably would be no campaign because the incumbent would have been locked in without any competition."

Although he did not mention Migden by name, Leno said holding office is a public trust and told the audience, "We need responsible representation."

Meanwhile, Migden is running on her legislative record, noting no other candidate in the race has carried more legislation that has become law.

"I am your senator. I love the job. I want to be rehired, and I think I have produced the results that the job requires," she said during the forum.

The last time a sitting lawmaker was defeated by a fellow party member, former Assemblyman Brian Setencich of Fresno lost the Republican primary because his association with Democratic Assembly Speaker Willie Brown angered party faithful.

Because Democrats outnumber Republicans 55 percent to 16 percent in the district, the winner of the Democratic primary will likely win the November general election.

Democratic senators have rallied around Migden, fearing that if Leno succeeds, other members of the Assembly will be emboldened to challenge them for their seats.

Senate President Pro Tem Don Perata has made her re-election a top priority. After Leno announced his intentions, only three of his bills cleared the Legislature last year and were signed by the governor, compared with 12 in 2006.

Tim Hodson, director of the Center for California Studies at California State University, Sacramento, said the motivation for "election-year politics" is obvious.

"People will say, 'Let's not have someone given the opportunity to look good,' " Hodson said.

Recently, the Senate passed a bill by Migden that would largely repeal a bill by Leno that gave rental car companies flexibility in advertising rental rates.

Migden's legislation raised eyebrows, but she said the law needed to be rewritten to spread costs among rental car companies and their customers.

During her appearance in Marin County she chose to talk about helping to end a property tax dispute with the state that returned millions of dollars to the county.

More than 50 percent of the voters in the district live in San Francisco, and it's been more than three decades since someone from the North Bay held the seat.

Migden also noted that she helped bring $400 million from the state to the North Bay to widen Highway 101, adding she made three calls a day to ensure the region got its fair share of bond money.

"I'm a fighter, I produce results, I don't quit," she said. "I make no apologies that sometimes it's a tough arena."

While Leno chairs the powerful Assembly Appropriations Committee, Migden was forced to step down as chairwoman of the Senate Appropriations Committee after she pushed the voting button of a Republican legislator while lobbying on the Assembly floor for one of her bills.

Nation, a late-comer to the race, is a moderate Democrat whose base is the North Bay. Political observers say his chances hinge on Leno and Migden splitting the San Francisco vote while he captures the suburbs.

A consultant on global warming and carbon market policies since he was termed out of the Assembly in 2006, Nation touts his ability to work with Republicans.

But Migden and Leno say Nation, whose $400,000 campaign chest has been largely financed by business groups seeking to limit consumer and malpractice lawsuits, is not liberal enough to represent the district.

Benefiting from their strong ties to labor groups, Migden and Leno have each raised and spent more than $1 million on the race.

Nation noted that during his time in the Assembly he worked on bipartisan legislation, including a bill that requires automakers to put labels on new cars with information about emissions produced by vehicles.

"We have different approaches to (universal) health care," he said. "Mine is very much like the Clinton-Obama plans at the national level a real plan that has real chance of passage."

The plan would spread financial responsibly among employers, workers and insurers. Leno and Midgen favor a government-run, single-payer system.

"There are a lot of things that we share in common, but there are a number of differences," Nation told the Marin County crowd.

They were lucky, he added, to have a choice.